Properly test range requests to file urls

[ferjm]

#22024 introduced a test to check the implementation of range requests to file urls. However, the test is not really testing that as it is naively using http urls instead :\ .

I tried changing this in #22134 with this patch. But, I couldn't figure out an easy way to make the update manifest tool ignore the generated resources.js file. So I am filing this issue to track the remaining work.

[jdm]

@jgraham what's the next step to investigate if adding a top-level .gitignore entry did not make a difference?