Allow running testharness/testdriver/reftests in servodriver (#34550)

* Make servodriver a thin wrapper over the base webdriver browser/executor classes.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Make ServoWebDriverRefTestExecutor a thin shell over the webdriver reftest executor.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Wait for the initial load to complete when opening a new tab via webdriver.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Remove assumption of a single tab from the webdriver server.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Serialize all keys of JS objects when converting to webdriver values.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Formatting.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Cleanup, docs, etc.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Use webview terminology more consistently.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Fix flake8 errors.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

tests/wpt/meta/MANIFEST.json

@@ -499057,7 +499057,7 @@
         []
        ],
        "servodriver.py": [
-        "2cb638be1569c91a963bf2cb3824f2c07788f8cc",
+        "1e9a2f3090ef1ff826dac4ca3a88a958242ffae6",
         []
        ],
        "webkit.py": [
@@ -499123,7 +499123,7 @@
         []
        ],
        "executorservodriver.py": [
-        "5d7d55f30b551f59bc0b16aacc8641c0fc24e39c",
+        "41b8ed9ac1891edb7ec332c61923314f8b1e5f19",
         []
        ],
        "executorwebdriver.py": [
@@ -499269,10 +499269,6 @@
        "d6616739e6ed63a79e8b2f5d8aee1d5b2ced7f49",
        []
       ],
-      "testharnessreport-servodriver.js": [
-       "7819538dbb8f4a807d5db2649c2540854996c865",
-       []
-      ],
       "testharnessreport-wktr.js": [
        "b7d350a4262cb6f0d38337b17311fea7bd73eb70",
        []