nrwt: should be able to run --platform test interactively

nrwt: should be able to run --platform test interactively
https://bugs.webkit.org/show_bug.cgi?id=76959

Reviewed by Adam Barth.

As part of refactoring the Port interfaces, in r103254 I made
passing a Host object to the Port mandatory; previously TestPort
objects would create their own (mock) hosts. However,
new-run-webkit-tests always passed a real Host, not a MockHost,
and so you couldn't run new-run-webkit-tests --platform test
interactively to debug test failures.

This change fixes that by creating a MockHost instead of a real
host if you say --platform test*.

* Scripts/webkitpy/layout_tests/run_webkit_tests.py:
(main):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105944 268f45cc-cd09-0410-ab3c-d52691b4dbfc