Add emerge --with-test-deps option (520652)

For packages matched by arguments, this option will pull in dependencies
that are conditional on the "test" USE flag, even if FEATURES=test is not
enabled for the matched packages. The state of the "test" USE flag is not
affected by this option. It only changes the effective dependencies.

X-Gentoo-Bug: 520652
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=520652