glsl/tests: Use exit code 126 to detect valgrind errors

valgrind returns exit code 126 if it can't write to the file passed to
--log-file. Hopefully it'll be the same for any other invalid valgrind
command line parameters or internal errors as well.

Using a different exit code (31) for this was hiding the fact that the
valgrind test wasn't actually working.

v2:
* Use exit code 126; can't treat any non-0 exit code as failure because
  glcpp is expected to exit with non-0 for some of the input we feed it

Reviewed-by: Dylan Baker <dylan.c.baker@intel.com> # v1
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9528>

src/compiler/glsl/glcpp/tests/glcpp_test.py

@@ -96,12 +96,12 @@ def _valgrind(glcpp, filename):
     extra_args = parse_test_file(contents, nl_format='\n')
 
     proc = subprocess.Popen(
-        ['valgrind', '--error-exitcode=31'] + glcpp + extra_args,
+        ['valgrind', '--error-exitcode=126'] + glcpp + extra_args,
         stdout=subprocess.DEVNULL,
         stderr=subprocess.PIPE,
         stdin=subprocess.PIPE)
     _, errors = proc.communicate(contents)
-    if proc.returncode != 31:
+    if proc.returncode != 126:
         return (True, [])
     return (False, errors.decode('utf-8'))