selftests: mptcp: userspace: refactor asserts

Instead of having a long list of conditions to check, it is possible to
give a list of variable names to compare with their 'e_XXX' version.

This will ease the introduction of the following commit which will print
which condition has failed (if any).

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>