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.

ChangeLog

@@ -1,3 +1,7 @@
+2018-04-19  Stefan Liebler  <stli@linux.vnet.ibm.com>
+
+	* scripts/test_printers_common.py (init_test): Disable lock elision.
+
 2018-04-19  Stefan Liebler  <stli@linux.vnet.ibm.com>
 
 	* math/test-tgmath.c (count_double, count_float,

scripts/test_printers_common.py

@@ -171,6 +171,9 @@ def init_test(test_bin, printer_files, printer_names):
     # Finally, load the test binary.
     test('file {0}'.format(test_bin))
 
+    # Disable lock elision.
+    test('set environment GLIBC_TUNABLES glibc.elision.enable=0')
+
 def go_to_main():
     """Executes a gdb 'start' command, which takes us to main."""