selftests/harness: Handle timeouts cleanly

When a selftest would timeout before, the program would just fall over
and no accounting of failures would be reported (i.e. it would result in
an incomplete TAP report). Instead, add an explicit SIGALRM handler to
cleanly catch and report the timeout.

Before:

	[==========] Running 2 tests from 2 test cases.
	[ RUN      ] timeout.finish
	[       OK ] timeout.finish
	[ RUN      ] timeout.too_long
	Alarm clock

After:

	[==========] Running 2 tests from 2 test cases.
	[ RUN      ] timeout.finish
	[       OK ] timeout.finish
	[ RUN      ] timeout.too_long
	timeout.too_long: Test terminated by timeout
	[     FAIL ] timeout.too_long
	[==========] 1 / 2 tests passed.
	[  FAILED  ]

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>