Speed up scsynth-based tests via Server.reset()

[josiah-wolf-oberholtzer]

Test modules with many tests relying on a running server could benefit from just resetting rather than rebooting between tests.

We'll need to:

• Create a module-scoped fixture effectively identical to the current server fixture
• Overwrite the server fixture with one that gets the module-scoped server and resets it after each use

Restructuring test modules to one-per-class rather than one-per-method will also shave off a lot of time, at the expense of some legibility (but also forcing us to name the tests better).