Implement `raw` flag

Some tests involving the directive prologue are invalidated by source
text transformations that insert executable code in the beginning of the
script. Implement a `raw` flag that allows these tests to opt-out of
this transformation. Update the relevant tests to use this flag (and
remove references to globals only available when code is injected).

Update the Python runner accordingly:

- Do not run tests marked as "raw" in strict mode
- Reject invalid test configurations

Update the browser runner accordingly:

- Do not modify the script body of tests marked as "raw"