regex-tests, regex: allow glibc re_search behavior

regex-tests, regex: allow glibc re_search behavior

The data passed to re_search by the test for glibc bug 15078 is a
multi-character collating element followed by a single character.
According to POSIX, "It is unspecified whether a non-matching list
expression matches a multi-character collating element that is not
matched by any of the expressions."

One of differences between glibc and gnulib implementations of
re_search is that glibc re_search matches multi-character
collating elements in that case while gnulib re_search doesn't.

Since both re_search implementations conform to standard, change
the test to allow glibc re_search behavior.

* tests/test-regex.c (main): In test for glibc bug 15078, reformat
re_search input data to make the multi-character collating element
in it clearly visible, and treat re_search return code 0 as valid.
* m4/regex.m4 (gl_REGEX): Likewise.