Auto merge of #15282 - dpyro:mach-format-single, r=wafflespeanut

Fixed mach/servo logger choice for single files

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Fixed mach/servo logger choice for single files as per @jdm’s [suggestion](https://github.com/servo/servo/issues/15193#issuecomment-275586263).

It’s also possible to do this by checking if the `test_list` path is a directory. This might be trying to make an implementation that is “too smart” though, so I stuck with checking the file extension against `[.html, .htm, .js, .xhtml]` as per the suggestion.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15193 (github issue number if applicable).

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