Auto merge of #15368 - szeged:get_availability, r=nox

Implement GetAvailability for Bluetooth

<!-- Please describe your changes on the following line: -->
This implements the [getAvailability](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetooth-getavailability) function from the spec.

---
<!-- 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
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/15368)
<!-- Reviewable:end -->

tests/wpt/mozilla/meta/MANIFEST.json

@@ -11443,6 +11443,24 @@
      {}
     ]
    ],
+   "mozilla/bluetooth/getAvailability/adapter-not-present.html": [
+    [
+     "/_mozilla/mozilla/bluetooth/getAvailability/adapter-not-present.html",
+     {}
+    ]
+   ],
+   "mozilla/bluetooth/getAvailability/adapter-off.html": [
+    [
+     "/_mozilla/mozilla/bluetooth/getAvailability/adapter-off.html",
+     {}
+    ]
+   ],
+   "mozilla/bluetooth/getAvailability/adapter-on.html": [
+    [
+     "/_mozilla/mozilla/bluetooth/getAvailability/adapter-on.html",
+     {}
+    ]
+   ],
    "mozilla/bluetooth/getCharacteristic/blocklisted-characteristic.html": [
     [
      "/_mozilla/mozilla/bluetooth/getCharacteristic/blocklisted-characteristic.html",
@@ -24372,6 +24390,18 @@
    "6bba458074ce7d8067b6420d4326cbb133e35304",
    "testharness"
   ],
+  "mozilla/bluetooth/getAvailability/adapter-not-present.html": [
+   "e4d5e516bee397ecefee1ef679f48f11b5dabe78",
+   "testharness"
+  ],
+  "mozilla/bluetooth/getAvailability/adapter-off.html": [
+   "0e3d7824d57ba676ae52c775fa21d8e6ec6d7073",
+   "testharness"
+  ],
+  "mozilla/bluetooth/getAvailability/adapter-on.html": [
+   "7e37c1b939712f5920b607e27a82f34573ca40fd",
+   "testharness"
+  ],
   "mozilla/bluetooth/getCharacteristic/blocklisted-characteristic.html": [
    "c75429728dedbc5059aa7c3cc72554bb6a9bbbcb",
    "testharness"