Add AT&T Research regex(3) regression testsuite

Before we replace our ancient regex, we need to baseline it.
This is a well-known and maintained regex testsuite from AT&T Research.
The following com

Add AT&T Research regex(3) regression testsuite

Before we replace our ancient regex, we need to baseline it.
This is a well-known and maintained regex testsuite from AT&T Research.
The following commands from tools/regression/lib/libc-regex can be used:

Make full-test-run (this runs all tests consecutively)

make test-basic
make test-categorize
make test-nullsubexpr
make test-leftassoc
make test-rightassoc
make test-forcedassoc
make test-repetition

To change the locale, set LOCALE (e.g. make LOCAL=en_US.UTF-8 test-basic)
These are the baseline results:

basic : TEST testregex, 539 tests, 0 errors
categorize : TEST testregex, 20 tests, 0 errors
nullsubexpr : TEST testregex, 84 tests, 31 errors
leftassoc : TEST testregex, 12 tests, 12 errors
rightassoc : TEST testregex, 24 tests, 0 errors
forcedassoc : TEST testregex, 48 tests, 8 errors
repetition : TEST testregex, 129 tests, 37 errors

UNSUPPORTED:
AUGMENTED,SHELL,CLASS_ESCAPE,COMMENT,DELIMITED,DISCIPLINE,ESCAPE,LEFT,
LENIENT,LITERAL,MINIMAL,MULTIPLE,MULTIREF,MUSTDELIM,NULL,RIGHT,
SHELL_DOT,SHELL_ESCAPED,SHELL_GROUP,SHELL_PATH,SPAN,regnexec,
regsubcomp,redecomp

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