selftests: mptcp: do complete cleanup at exit

After commit 05be5e27 ("selftests: mptcp: add disconnect tests")
the mptcp selftests leave behind a couple of tmp files after
each run. run_tests_disconnect() misnames a few variables used to
track them. Address the issue setting the appropriate global variables

Fixes: 05be5e27 ("selftests: mptcp: add disconnect tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>