Auto merge of #15665 - UnICorN21:ds_store, r=SimonSapin

fixed an issue related with .DS_Store

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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 #15581.

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

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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")