mscoree/tests: Avoid temporary directory name collisions.

mscoree/tests: Avoid temporary directory name collisions.

test_loadpaths_execute() was sometimes getting ERROR_ALREADY_EXISTS when
creating its temporary directoryi. That was probably because
GetTempFileNameW() only uses the low 16-bits of the 'random' LUID which
goes over a wider range, potentially leading to collisions in the lower
16-bits.
Avoid AllocateLocallyUniqueId() entirely and add a helper function to
create a guaranteed new directory.
Also try to use the current directory first as it is assumed to have
been configured to ward off anti-virus programs (specifically
Microsoft Defender) which is especially important when writing
executables to that location. Only switch to the system's temporary
directory if the current directory is not a writable location.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=54560