Issue 11758: increase xml.dom.minidom test coverage (contributed by mdorn, reviewed by Sandro Tosi).

Patch by Akira Kitada.

On FreeBSD6, pthread_kill() doesn't work on the main thread before the creation
of the first thread. Create therefore a dummy thread (no-op) a startup to
initialize the pthread library.

Add also a test for this use case, test written by Charles-François Natali.

Patch by Francisco Martín Brugué.

The new wording is based on the comments in the code, which
match the actual behavior.

Locks are implemented using a mutex and a condition variable of the pthread
library on FreeBSD6. POSIX condition variables cannot be interrupted by signals
(see pthread_cond_wait manual page).

I didn't rename the variable because I can see no good reason
to break backward compatibility just to put an underscore in
the name.

Cannot test an empty environment on Windows: Windows requires at least the
SYSTEMROOT environment variable to start Python.