bug#50583: 27.2; test-completion should return non-nil

[Kevin Vigouroux via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

In the following code, I assume that `test-completion' should return a
non-‘nil’ value because "foo" is a valid completion alternative.

(test-completion
 "foo"
 '("foobar" "bar" "barfoo"))

=> nil

(try-completion
 "foo"
 '("foobar" "bar" "barfoo"))

=> "foobar"

#+begin_quote 
-- Function: test-completion string collection &optional predicate
     This function returns non-‘nil’ if STRING is a valid completion
     alternative specified by COLLECTION and PREDICATE.  The arguments
     are the same as in ‘try-completion’.  For instance, if COLLECTION
     is a list of strings, this is true if STRING appears in the list
     and PREDICATE is satisfied.

     This function uses ‘completion-regexp-list’ in the same way that
     ‘try-completion’ does.

     If PREDICATE is non-‘nil’ and if COLLECTION contains several
     strings that are equal to each other, as determined by
     ‘compare-strings’ according to ‘completion-ignore-case’, then
     PREDICATE should accept either all or none of them.  Otherwise, the
     return value of ‘test-completion’ is essentially unpredictable.

     If COLLECTION is a function, it is called with three arguments, the
     values STRING, PREDICATE and ‘lambda’; whatever it returns,
     ‘test-completion’ returns in turn.
#+end_quote


In GNU Emacs 27.2 (build 1, aarch64-unknown-linux-gnu, GTK+ Version 3.24.27, cairo version 1.17.4)
 of 2021-03-26 built on leming
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Manjaro ARM

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit
Mark set [2 times]
Making completion list...
Quit
nil
"foobar"
nil
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-wide-int
 --with-modules --with-cairo --with-harfbuzz 'CFLAGS=-march=armv8-a -O2
 -pipe -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

Important settings:
  value of $LC_MONETARY: fr_FR.UTF-8
  value of $LC_NUMERIC: fr_FR.UTF-8
  value of $LC_TIME: fr_FR.UTF-8
  value of $LANG: fr_FR.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug modus-operandi-theme pcase modus-themes
gnus-group gnus-undo gnus-start gnus-cloud nnimap nnmail mail-source
utf7 netrc nnoo parse-time iso8601 gnus-spec gnus-int gnus-range
gnus-win smtpmail sendmail message rmc puny dired dired-loaddefs
format-spec rfc822 mml mml-sec epa derived epg epg-config mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
gnus nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045 ietf-drums
text-property-search time-date mail-utils mm-util mail-prsvr wid-edit
info package easymenu browse-url url-handlers url-parse auth-source
cl-seq eieio eieio-core cl-macs eieio-loaddefs password-cache json
subr-x map url-vars seq byte-opt gv bytecomp byte-compile cconv
cl-loaddefs cl-lib tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame minibuffer cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
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cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 163236 36337)
 (symbols 48 13579 1)
 (strings 32 33525 2404)
 (string-bytes 1 1125620)
 (vectors 16 17161)
 (vector-slots 8 210976 13280)
 (floats 8 159 47)
 (intervals 56 372 69)
 (buffers 1000 13))