t/t4018: test whether the word_regex patterns compile

Previously (e3bf5e4), 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 <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018-diff-funcname.sh

@@ -40,6 +40,11 @@ do
 		! ( git diff --no-index Beer.java Beer-correct.java 2>&1 |
 			grep "fatal" > /dev/null )
 	'
+	test_expect_success "builtin $p wordRegex pattern compiles" '
+		! ( git diff --no-index --word-diff \
+			Beer.java Beer-correct.java 2>&1 |
+			grep "fatal" > /dev/null )
+	'
 done
 
 test_expect_success 'default behaviour' '