selftests/timers: Quiet warning due to lack of return check on brk

The posix_timers.c test has a loop that tries to keep it in
kernel space, repeatedly calling brk(). However, it doesn't
check the return value, which causes warnings.

This patch adds a err value which captures the return value
and modifies the test so it will quit if a failure occurs.

Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Tested-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>