drd/tests/swapcontext: Add SIGALRM handler to avoid stacktrace

drd/tests/swapcontext: Add SIGALRM handler to avoid stacktrace

During testing for oe-core build on QEMU,
SIGALRM can trigger during nanosleep.
This results a different stderr output than expected.

```
==277== Process terminating with default action of signal 14 (SIGALRM)
==277==    at 0x36C74C3943: clock_nanosleep@@GLIBC_2.17 (clock_nanosleep.c:43)
==277==    by 0x36C74C8726: nanosleep (nanosleep.c:25)
```

This stacktrace printing will not occur
if we add a handler that simply exits.

https://bugs.kde.org/show_bug.cgi?id=435160

Signed-off-by: Yi Fan Yu <yifan.yu@windriver.com>