testutils.c likes to print summaries after a test completes,
including if it failed.  But if the test outright exit()s,
this summary is skipped.  Enforce that we return instead of exit.

* cfg.mk (sc_prohibit_exit_in_tests): New syntax check.
* tests/commandhelper.c (main): Fix offenders.
* tests/qemumonitorjsontest.c (mymain): Likewise.
* tests/seclabeltest.c (main): Likewise.
* tests/securityselinuxlabeltest.c (mymain): Likewise.
* tests/securityselinuxtest.c (mymain): Likewise.
* tests/testutils.h (VIRT_TEST_MAIN_PRELOAD): Likewise.
* tests/testutils.c (virtTestMain): Likewise.
(virtTestCaptureProgramOutput): Use symbolic name.