test_user_pass: Check fatal errors for empty username/password

test_user_pass: Check fatal errors for empty username/password

Required a fix to mock_msg to make tests of M_FATAL
possible at all.
This also tests some cases which arguably should throw
a fatal error but do not.

v2:
 - Suppress LeakSanitizer errors for fatal error tests.
   Due to aborting the function, the memory will not be
   cleaned up, but that is expected.
v3:
 - Disable assert tests with MSVC. Does not seem to catch
   the error correctly.
 - Rebase on top of parallel-tests series to get
   AM_TESTS_ENVIRONMENT.
v8:
 - Update srcdir handling according to master.
v10:
 - Update mock_msg.c fatal handling to be compatible
   with NO_CMOCKA.

Change-Id: Icabc8acf75638c86c8c395e9ffecba7a7226cd97
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/474
Message-Id: <20251010211154.2780-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59245149/
Signed-off-by: Gert Doering <gert@greenie.muc.de>