bug#30807: Tramp tests take waaayyy too long and refuse to exit

[Paul Eggert <eggert@cs.ucla.edu>]

For the past few weeks "make check" has been taking a looooong time on 
Emacs master on my Fedora 27 desktop (AMD Phenom II X4 910e). The first 
line of "make" output that causes a long delay is:

   GEN      lisp/net/tramp-tests.log

and some of the tests in the corresponding script look to be very slow. 
Can this be sped up? Let's mark the slow tests to be expensive, so that 
they are run only if one executes "make check-expensive".

Worse, there's another long wait here:

   GEN      lisp/net/tramp-archive-tests.log

after this line is output:

    passed  20/22  tramp-archive-test40-file-system-info

and here the wait is so long that I gave up trying to run the tests and 
typed control-C to exit them. Presumably the semi-inflooping test is 
tramp-archive-test42-auto-load. (Why is it 42 and not 41? Why are there 
two test42s? I don't know.)

Worse yet, after I control-C out of "make check" and get a shell prompt 
back, a subsidiary Emacs that is running the Tramp test refuses to exit 
and continues to chew up CPU time in the background until I use "kill 
3304" (or whatever) to kill it. This really needs to get fixed.

Because of these problems I no longer routinely run "make check".