Disable lock elision for mutex pretty printer tests.

If e.g. the testcase nptl/test-mutex-printers is run
with enabled lock-elision, it fails on s390x with:
Error: Response does not match the expected pattern.
Command: print *mutex
Expected pattern: pthread_mutex_t
Response:  No symbol "mutex" in current context.
(gdb)

See https://www.sourceware.org/ml/libc-alpha/2018-03/msg00583.html
for more details.

In fact the mutex pretty printer tests rely on looking at the
internal details of the lock, thus we disable it by setting up
the GLIB_TUNABLES environment variable inside gdb.

ChangeLog:

	* scripts/test_printers_common.py (init_test): Disable lock elision.