Auto merge of #27169 - alarsyo:mach-test-cargo-target-dir, r=jdm

Mach test cargo target dir

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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

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- [X] These changes do not require tests because they're about tooling

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This change allows launching tests this way:

```sh
CARGO_TARGET_DIR=other-dir ./mach test-wpt ...
```

The build and check commands already check for the presence of the `CARGO_TARGET_DIR` variable, but that wasn't the case for wpt tests. This resulted in an error about the `servo` executable not being found in `target/debug/servo`.

tests/wpt/run.py

@@ -4,6 +4,7 @@
 
 import multiprocessing
 import os
+from os import path
 import sys
 import mozlog
 import grouping_formatter
@@ -72,7 +73,10 @@ def set_defaults(kwargs):
         bin_name = "servo"
         if sys.platform == "win32":
             bin_name += ".exe"
-        bin_path = servo_path("target", bin_dir, bin_name)
+        if "CARGO_TARGET_DIR" in os.environ:
+            bin_path = path.join(os.environ["CARGO_TARGET_DIR"], bin_dir, bin_name)
+        else:
+            bin_path = servo_path("target", bin_dir, bin_name)
 
         kwargs["binary"] = bin_path
         kwargs["webdriver_binary"] = bin_path