Fix accessing regex-resources in out-of-tree test runs in regex-tests

Fix accessing regex-resources in out-of-tree test runs in regex-tests

[82a487d: Fix reading of regex-resources in regex-tests] attempted to
fix regex-tests failing when run from the source tree (i.e. via make)
by hard-coding path to regex-resources directory relative to the test
directory.

This fixed runs from the tree but broke the test when run using other
methods.

Fix by trying ‘load-file-name’ or ‘buffer-file-name’, whichever is set.

* test/src/regex-tests.el (regex-tests--resources-dir): New variable
storing path to the regex-resources directory.
(regex-tests-generic-line): Use aforementioned variable.