[RFA]: threaded watchpoint test

[Andrew Cagney]

> (1) The test suite does not keep its own info.  The test suite asks
>     gdb whether gdb supports feature X, and trusts gdb.
> 
> (2) The test suite keeps separate info.  That way, the test suite
>     is actually testing gdb, not just asking and trusting it.
>     But the info can get out of sync.
> 
>     (2A) A whitelist of systems to test.
> 
>     (2B) A blacklist of systems not to test.

We need both:

- the testsuite asks and trusts GDB if feature X is supported

- cross-check test confirming that GDB matches theory

That way as each feature is implemented, the testsuite's coverage 
expands accordingly.  But, if there's a screwup, the cross-check detects 
the problem.

Andrew