Allow running conformance tests in parallel

[Scott Ritchie]

Dan Kegel wrote:
> About the same number of tests fail regardless of -j value.
> 
> How it works:
> this patch adds a new function, winetest_exclusive(),
> that waits until it can acquire an exclusive lock.
> It should be called near the top of the START_TEST body
> for any test that needs exclusive access to the
> system.  (In tests that exec themselves, the call has to
> be placed carefully to avoid having the child call it.)
> I did this for all tests that call CreateWindow, and
> a few more that seemed like they might need it.
> 
> The list of tests marked exclusive is somewhat rough;
> probably a few tests should be added or removed,
> but it's pretty good for a first cut.
> 
"About" -- shouldn't it be exactly?  Unless tests are sometimes supposed
fail inconsistently, then you probably need it on every one that behaves
differently.

Thanks,
Scott Ritchie