libc-test: Run as non-root user

libc-test: Run as non-root user

Some of tests impose rlimit on it before running which wont be imposed
when running as root user.

Fixes
src/regression/pthread_atfork-errno-clobber.c:23: (pid = fork()) == -1 failed: fork succeeded despite rlimit
src/regression/pthread_atfork-errno-clobber.c:23: (pid = fork()) == -1 failed: fork succeeded despite rlimit
FAIL src/regression/pthread_atfork-errno-clobber-static.exe [status 1]

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>