Split regex character class test into smaller chunks

Split regex character class test into smaller chunks

Having one test for all character classes it is not always trivial to
determine which class is failing.  This happens when failure is caused
by ‘(should (equal (point) (point-max)))’ not being met.

With per-character class tests, it is immidiatelly obvious which test
causes issues plus tests for all classes are run even if some of them
fail.

* test/src/regex-tests.el (regex-character-classes): Delete and split
into…
(regex-tests-alnum-character-class, regex-tests-alpha-character-class,
regex-tests-ascii-character-class, regex-tests-blank-character-class,
regex-tests-cntrl-character-class, regex-tests-digit-character-class,
regex-tests-graph-character-class, regex-tests-lower-character-class,
regex-tests-multibyte-character-class,
regex-tests-nonascii-character-class,
regex-tests-print-character-class, regex-tests-punct-character-class,
regex-tests-space-character-class,
regex-tests-unibyte-character-class,
regex-tests-upper-character-class, regex-tests-word-character-class,
regex-tests-xdigit-character-class): …new tests.