selftests: mptcp: Rename wait function

The "selftests: mptcp: improve 'fair usage on close' stability" commit
changed that self test to check the TcpAttemptFails MIB instead of
looking for TW sockets. The associated bash function wasn't renamed in
that commit because of the merge conflicts it would cause, so this
commit updates the function name as Paolo originally intended.

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