test: don't treat a signal exit as success

WEXITSTATUS() "should be employed only if WIFEXITED returned true", see
wait(2). If a test failed with an abort, WIFEXITED is false and WEXITSTATUS
is... undefined? and apparently zero, so test case failures would cause a
false postive test result.

This doesn't affect a normal test run because check handles the aborts
correctly, but the valgrind invocation with CK_FORK ended up being handle by
litest. So with the result that any abort during valgrind was a silent success
and if there was a memleak in the same process that exited with a signal, the
memleak would be ignored too.

Fixes #267

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>