Fix posix/tst-regex by using UTF-8 and own test input

Problem reported by Stefan Liebler in:
https://sourceware.org/ml/libc-alpha/2019-08/msg00658.html
* posix/tst-regex.c: Convert this file from Latin-1 to UTF-8.
(do_test, test_expr): Adjust to the fact that this source file,
and the test data in ChangeLog.8, is now UTF-8 instead of Latin-1.
* posix/tst-regex.input: Copy from ChangeLog.old/ChangeLog.8,
so that it is now UTF-8.