It can be useful to be able to write a single-threaded test but run it
as part of a multi-threaded program simply to exercise glibc
synchronization code paths, e.g. the malloc implementation.
This commit adds support to enable this kind of testing. Tests that
define TEST_IN_THREAD, either as TEST_THREAD_MAIN or TEST_THREAD_WORKER,
and then use support infrastructure (by including test-driver.c) will be
accordingly run in either the main thread, or in a second "worker"
thread while the other thread waits.
This can be used in new tests, or to easily make and run copies of
existing tests without modifying the tests themselves.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
