Check errno on startup in timerfd__{reset_to_very_long,unmodified_errno}

Due to a bug in CheriBSD, errno was set non-zero when starting this test,
but without these checks the test confusingly failed in the later check
which made me think something was wrong in timerfd_create. Check errno on
startup to make this more obvious.

See CTSRD-CHERI/cheribsd#1424

test/timerfd-test.c

@@ -900,6 +900,7 @@ ATF_TC_BODY_FD_LEAKCHECK(timerfd__short_evfilt_timer_timeout, tc)
 ATF_TC_WITHOUT_HEAD(timerfd__unmodified_errno);
 ATF_TC_BODY_FD_LEAKCHECK(timerfd__unmodified_errno, tc)
 {
+	ATF_REQUIRE(errno == 0);
 	int timerfd = timerfd_create(CLOCK_MONOTONIC, /**/
 	    TFD_CLOEXEC | TFD_NONBLOCK);
 	ATF_REQUIRE(timerfd >= 0);
@@ -911,6 +912,7 @@ ATF_TC_BODY_FD_LEAKCHECK(timerfd__unmodified_errno, tc)
 			    .it_value.tv_nsec = 100000000,
 			},
 			NULL) == 0);
+	ATF_REQUIRE(errno == 0);
 	(void)wait_for_timerfd(timerfd);
 	ATF_REQUIRE(errno == 0);
 
@@ -937,6 +939,7 @@ ATF_TC_BODY_FD_LEAKCHECK(timerfd__unmodified_errno, tc)
 ATF_TC_WITHOUT_HEAD(timerfd__reset_to_very_long);
 ATF_TC_BODY_FD_LEAKCHECK(timerfd__reset_to_very_long, tc)
 {
+	ATF_REQUIRE(errno == 0);
 	int timerfd = timerfd_create(CLOCK_MONOTONIC, /**/
 	    TFD_CLOEXEC | TFD_NONBLOCK);
 	ATF_REQUIRE(timerfd >= 0);