Also, if you set a relative path in native-comp-eln-load-path (directly,
not via `startup-redirect-eln-cache'), it defaults to
`invocation-directory', which is (in an in-place run) the src/ subdir.

Which is weird:

(push "./test-native" native-comp-eln-load-path)

and then the test-native/ subdir is created both under
`user-emacs-directory' and `invocation-directory' (the .eln files go to the
second one).