local run-dir mode for sub-suites

[hjoliver]

Supersedes #2749 - specifically, to address this comment: #2749 (comment)

I've figured out why cylc run /path/to/suite.rc unexpectedly worked, and resulted in a run directory inside the source directory, and I have a branch that intentionally supports this behaviour - which will (I think) undoubtedly be useful for sub-suites (even though we may have more complete solutions for this sort of thing longer term).

(Note this is a partial solution to old issue #171 too).

[oliver-sanders]

To add to the use cases, it looks like I was taking advantage of this feature to test a tutorial suite.