Delay launchdir init until test execution

[jmuhlich]

This change separates the definition of the launch/meta/output/work directory paths from the physical re-initialization of the actual directories on the filesystem. The definition of the paths still happens up front, but the init and the rest of the setup() call has been moved down into the test execution engine inner loop. This prevents nf-test from changing the filesystem until the last moment before any given test actually runs. With this change, nf-test list or running specified tests no longer deletes the output from other tests. This also conveniently enables running tests in parallel without any extra filesystem isolation or containerization.

Fixes #213

[lukfor]

Thanks 🙏 I will go over the code and will merge it in the next few days 🚀