Exclude external-http tests when running `phpunit`.

The external-http tests are very slow, and `Wp_Http` functionality is fairly
isolated, so the benefits of skipping these tests by default outweigh the
risks.

A `grunt phpunit:external-http` subtask has been added, to ensure that the
tests are executed during exhaustive runs of the test suite, such as in
continuous integration.

Fixes #30304.

git-svn-id: https://develop.svn.wordpress.org/trunk@30298 602fd350-edb4-49c9-b593-d223f7449a82