Restrict downcasing in elisp xref tests (bug#25534)

Restrict downcasing in elisp xref tests (bug#25534)

The tests happen to not fail at the moment because find-library-name
now has an extra feature, find-library--from-load-history, which
happens to do a case-insensitive regexp match; but still it seems
better not to rely on this.

* test/lisp/progmodes/elisp-mode-tests.el (xref--case-insensitive):
New variable.
(xref-elisp-test-run, emacs-test-dir): Only downcase if the
filesystem seems to be case-insensitive.