selftests: mptcp: userspace: print error details if any

Before, only '[FAIL]' was printed in case of error during the validation
phase.

Now, in case of failure, the variable name, its value and expected one
are displayed to help understand what was wrong.

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>