servo: Merge #15665 - fixed an issue related with .DS_Store (from UnICorN21:ds_store); r=SimonSapin

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---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15581.

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: 7adc79047dc1fafef3615cb8fcc8e4a43764f262

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 2e49175de83bee2706ee876b4f3086025c49f5e3

servo/python/servo/testing_commands.py

@@ -232,7 +232,7 @@ class MachCommands(CommandBase):
                 test_patterns.append(test)
 
         if not packages:
-            packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit")))
+            packages = set(os.listdir(path.join(self.context.topdir, "tests", "unit"))) - set(['.DS_Store'])
 
         packages.discard('stylo')
 
@@ -345,7 +345,7 @@ class MachCommands(CommandBase):
                 test_patterns.append(test)
 
         if not packages:
-            packages = set(os.listdir(path.join(self.context.topdir, "tests", "compiletest")))
+            packages = set(os.listdir(path.join(self.context.topdir, "tests", "compiletest"))) - set(['.DS_Store'])
 
         packages.remove("helper")