Auto merge of #29934 - mrobinson:move-third-party, r=atbrakhi

Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

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---
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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 do not require tests because this should not change behavior.

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python/servo/testing_commands.py

@@ -339,8 +339,8 @@ class MachCommands(CommandBase):
 
         if all or tests:
             print("Running WebIDL tests...")
-            test_file_dir = path.abspath(path.join(PROJECT_TOPLEVEL_PATH, "components", "script",
-                                                   "dom", "bindings", "codegen", "parser"))
+
+            test_file_dir = path.abspath(path.join(PROJECT_TOPLEVEL_PATH, "third_party", "WebIDL"))
             # For the `import WebIDL` in runtests.py
             sys.path.insert(0, test_file_dir)
             run_file = path.abspath(path.join(test_file_dir, "runtests.py"))