Support running the tests in parallel

[patrickfreed]

The test suite currently uses some global variables for things like the mocked home directory and capturing output. This has the unfortunate side effect that the tests must be run sequentially. We should modify such variable usage to be associated with an instance of swiftly such that the tests can be run in parallel. Much of the time in the tests is spent on downloading toolchains, so this may result in a significant speedup.