Cygwin: Ensure temporary directory used by tests exists

Cygwin: Ensure temporary directory used by tests exists

By default, libltp tests will create temporary files in a subdirectory
of /tmp, which will (nowadays) be located relative to the test DLL (by
assuming that it is in /bin).  This will evaluate to the directory
$target_builddir/winsup/tmp, which doesn't exist.

The location used for these temporary files can be explicitly controlled
by setting the TDIRECTORY env var.  Arrange to set that env var to the
/cygdrive path of a tmp subdirectory of the build directory.

Unfortunately, libltp doesn't clean the temporary directory if
TDIRECTORY is set, and some tests assume they are started in a clean
directory, so we need to do that in tcl.