Use the new coordinated_forks option coming in Test::Builder 1.5.0_7

See https://github.com/schwern/test-more/pull/367 and the issue/334 branch at
https://github.com/schwern/test-more/  1.5.0 alpha 7 will be released soon with this feature.

All tests pass, even subtests.

PS I'd like to see this released ASAP to unblock testing Test::TCP and all its dependencies.  I'm happy to make the release myself.