Honour *.headers (including *.sub.headers and __dir__{.sub}.headers)
when serving tests automatically generated from *.js files. The
documentation doesn't mention this exception and reasonable people would
assume *.headers work the same way as with static files. Add a
functional test, too.

Fixes https://crbug.com/1135559.

Drive-by: some stylistic changes and docstrings.

Co-authored-by: Stephen McGruer <[email protected]>