Command line parser for test harness

[nsubtil]

The test harness functionality desperately needs a command line parser. We need to be able to do things such as:

• Run one specific test by name
• Run a set of tests (see #35)
• Run tests from a pre-defined list
• Enable/disable compute devices on the fly (e.g., run the CPU path only, or pick one specific GPU out of a multi-GPU setup)

The exact set of features that we'll implement on top of this is TBD, but they all require some way to receive runtime parameters, hence the need for a command line parser of some sort.

[nsubtil]

Running a specific test and enabling/disabling compute devices is mostly implemented. There's no option to pick a specific GPU, but setting CUDA_VISIBLE_DEVICES should be an adequate proxy for now.

Lowering this to P2 for the remaining features.