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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tests/wpt/run.py

@@ -32,7 +32,13 @@ def run_tests(paths=None, **kwargs):
     mozlog.commandline.log_formatters["servo"] = \
         (grouping_formatter.GroupingFormatter, "A grouping output formatter")
 
+    use_mach_logging = False
     if len(kwargs["test_list"]) == 1:
+        file_ext = os.path.splitext(kwargs["test_list"][0])[1].lower()
+        if file_ext in [".htm", ".html", ".js", ".xhtml"]:
+            use_mach_logging = True
+
+    if use_mach_logging:
         wptrunner.setup_logging(kwargs, {"mach": sys.stdout})
     else:
         wptrunner.setup_logging(kwargs, {"servo": sys.stdout})