Add emerge --with-test-deps option for bug #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
which are processed by the depgraph._add_pkg_deps method.

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