optionally skip tests when installing dependencies

[gkleen]

When installing dependencies with sbin/rt-test-dependencies, by default all installed dependencies are tested.
This PR does not change the default behaviour.

This PR adds a flag to allow the user to opt-out of having the installed dependencies tested.
This has proved necessary for our deployment; not alle dependencies document the additional dependencies/environment they require for their tests to pass. Also, testing everything slowed down our automated installation considerably.