lib/locale.t: Untaint before checking if next thing taints

The tests weren't testing what they purported to, as we should be sure
to start with untained values to see if the operation taints.
lib/locale.t: Add some test names
Make taint checking regex compile time instead of runtime

See discussion at https://rt.perl.org/Ticket/Display.html?id=120675
There are several unresolved  items in this discussion, but we did agree
that tainting should be dependent only on the regex pattern, and not the
particular input string being matched against:

"The bottom line is we are moving to the policy that tainting is based
on the operation being in locale, without regard to the particular
operand's contents passed this time to the operation. This means simpler
core code and more consistent tainting results. And it lessens the
likelihood that there are paths in the core that should taint but don't"

This commit does the minimal work to change regex pattern matching to
determine tainting at pattern compilation time.  Simply put, if a
pattern contains a regnode whose match/not match depends on the run-time
locale, any attempt to match against that pattern will taint, regardless
of the actual target string or runtime locale in effect.  Given this
change, there are optimizations that can be made to avoid runtime work,
but these are deferred until later.

Note that just because a regular expression is compiled under locale
doesn't mean that the generated pattern will be tainted.  It depends on
the actual pattern.  For example, the pattern /(.)/ doesn't taint
because it will match exactly one character of the input, regardless of
locale settings.