sorbet: Handle `before` blocks using `let` values from within tests

- Enable typechecking of tests where a top-level `before` or `subject`
  block references a value that is only defined via `let` in tests,
  which Sorbet cannot resolve statically.
- The actual value of the `let` that matters for the test and its setup
  is set in the individual tests, which will always override the
  top-level `let` value that we set.