kselftests: timers: set-timer-lat: Tweak reporting when timer fires early

Rather than printing an error inside the alarm signal handler, set a
flag that we check later.  This keeps the test from spamming the console
every time the alarm fires early.  It also fixes the test exiting with
error code 0 if this was the only test failure.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>