Fix reading of regex-resources in regex-tests

Fix reading of regex-resources in regex-tests

* test/src/regex-tests.el (regex-tests-generic-line): Referring to
‘buffer-file-name’ does not work when running the test from command
line, i.e. via make, which results in (wrong-type-argument stringp nil)
failures.  Replace it with hard-coded path.
(regex-tests-BOOST, regex-tests-PCRE, regex-tests-PTESTS-whitelist,
regex-tests-TESTS-whitelist): ‘regex-tests-generic-line’ now  includes
the ‘regex-resources’ path component so the tests don’t need to specify
it explicitly.