Weird header behavior with *.js tests and CSP

[yoavweiss]

While working on a test with multiple different CSP policies and trying to move it to be a JS test, it seemed like only some of the provided headers (in the corresponding .headers) file were taken into account, causing the test to fail, as only some of the policies were in place.

I debugged it a bit, but haven't reached full understanding of the underlying issue.

Are *.js tests being redirected somehow to the equivalent *.html test variant?