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.
1 job for emacs-26 in 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #328


Name Stage Failure
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.4.2 (cf91d5e1)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:4879790bd60d439cfe39c063660eef7af525d5f6f1cbb701a14c7cfc11cbfcf7 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: open /var/run/docker/libcontainerd/containerd/ea21ee0bd9b79d69c8020d4fedd1c71f58d967a7ca6aaf1e52b9016457123e25/init/shim-log.json: no such file or directory (executor_docker.go:1015:0s)