qa: update test cases to check for ENOTDIR instead of EINVAL

qa: update test cases to check for ENOTDIR instead of EINVAL

- test_nfs_export_creation_at_filepath:
ENOTDIR is raised instead of EINVAL which is better
aligned with the nature of the failure

- test_nfs_export_creation_at_symlink:
ENOTDIR is raised instead of ENOENT since the code
can now check if the path is symlink but won't follow
it.

Signed-off-by: Dhairya Parmar <dparmar@redhat.com>