(QE-43) handle empty pre-suite/tests/post-suite options

Raise error on empty pre-suite/post-suite/tests options - but also allow for running without those options (the harness will just skip pre-suite/post-suite/tests phases and continue as normal).

Remove default value for '--tests'.

Now --install-only behaves the same as not specifying --tests - might be worth removing the option for succinctness.