tests: zms: add function to test zms handling of invalid inputs

The ZMS API should not unexpectedly crash the application; this commit
adds tests to check if the ZMS API can safely handle and return -EINVAL
if called with a NULL fs pointer.

This commit also adds test to confirm the return value matches the
documentation if user calls the ZMS API before ZMS is mounted

Signed-off-by: Theis Mejnertsen <theismejnertsen@gmail.com>