seccomp: mmap test results depend on kernel/libseccomp/glibc

seccomp: mmap test results depend on kernel/libseccomp/glibc

Like with shmat already the actual results of the test
test_memory_deny_write_execute_mmap depend on kernel/libseccomp/glibc
of the platform it is running on.

There are known-good platforms, but on the others do not assert success
(which implies test has actually failed as no seccomp blocking was achieved),
but instead make the check dependent to the success of the mmap call
on that platforms.

Finally the assert of the munmap on that valid pointer should return ==0,
so that is what the check should be for in case of p != MAP_FAILED.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>