t/t4018: test whether the word_regex patterns compile

t/t4018: test whether the word_regex patterns compile

Previously (e3bf5e43), a test was added to test whether the builtin
xfuncname regular expressions could be compiled without error by regcomp.
Let's do the same for the word_regex patterns.  This should help catch any
cross-platform incompatibilities that exist between the pattern creator's
system and the various platforms that the test suite is commonly run on.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>