Add a test for minijail_reset_signal_mask()

This change adds a test for minijail_reset_signal_mask(). It also
changes EXPECT_... to ASSERT_... when fork(2)ing because EXPECT doesn't
cause abnormal process termination.

Bug: None
Test: make tests
Change-Id: If95543704b1cc79aec3b5c4ce51c55e444faccb3
1 file changed