posix_signal test: increase tolerance on ALRM

On some systems, the previous test tolerance of 10 ms fails, e.g.:

Entering test case "test_alarm"
wait alarm(1)
[26-02-2013 02:44:26.671134] [WARNING] process received signal ALRM
.../test/unit/utility/posix_signal.cpp(119): error in "test_alarm": difference{0.0182504} between timer.elapsed(){1.018250436} and 1.{1} exceeds 0.01
Leaving test case "test_alarm"