Also update BUILD.txt

When CK_FORK is set to 'no' the fixtures are executed for every new test
inside of the same process. Global variables must be set to the expected
values by the fixtures.

check_and_open-tests.c: the filename template for mkstemp() was a
globally defined character string. After the first call to mkstemp() the
trailing XXXXXX are substituted by random values, a second call to
mkstemp() with this character string fails. This patch initialize the
filename template before mkstemp() is called with the help of strdup()
and the memory is freed in the teardown fixture.

krb5_utils-tests.c: this patch sets the just freed global talloc context
to NULL to make a consistency check in the setup fixture pass.

The leak checking code which is used by the resolve and failover tests
frees talloc's autofree context which is not recommended. As a
consequence the initialization of tevent failed when it was called by
the second test and CF_FORK=no, because it holds some data in the
autofree context.

This patch introduces a global talloc context which should be uses by
the test as the root of their memory hierarchy instead of NULL. This
global context is used in the leak checking routines.

Not all types of memory leaks can be detected by the new version , it is
recommended to use valgrind or similar tools additionally.