test(slow): simplify bootstrapping tests

[erickzhao]

A little re-organization of our slow tests to minimize CI time (reduces the number of tests a bit) and make the tests easier to read.

1. Instead of running a describe.each loop for each supported package manager, run a single describe block asserting the expected characteristics of each (node_modules is in expected format and lockfile is correct).
2. Move all other tests outside of this
3. Get rid of beforeInitTest and instead call api.init in each test so it's more intuitive. Vitest timings become more accurate because api.init time is accounted for within each test as well.
4. Separate bootstrap tests into separate import and init files.