UserAgent-specific files in Web Platform Tests

Hi all,

Some context: We, the Web Bluetooth team, are looking into upstreaming our
Chromium Layout Tests to Web Platform Tests. In order to test the Web
Bluetooth API, we are introducing a Test API that accompanies the spec and
allows our tests to fake Bluetooth Devices: Web Bluetooth Test
<https://docs.google.com/document/d/1Nhv_oVDCodd1pEH_jj9k8gF4rPGb_84VYaZ9IG8M_WY/edit#heading=h.ap8dnjfog4qc>
..

Parts of this API are implemented in JS. These parts are Chromium specific,
e.g. how to talk with our IPC system, so it wouldn't make sense to include
them as resources.

To that extent, we would like to add a file called "web-bluetooth-test.js"
which would be similar to "testharnessreport.js" to the testharness repo.
On the main repo that file would be empty but on the Chromium repo that
file would have the necessary code to fake devices in Chromium.

There are many APIs that follow a similar pattern: they define a Test API
surface that they use to fake behavior. Some examples include Geolocation
<https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/geolocation-api/error.html?type=cs&q=mojo-helpers+file:%5Esrc/third_party/WebKit/LayoutTests/geolocation-api/+package:%5Echromium$&l=17>,
Vibration
<https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/vibration/vibration-durations.html?l=13>,
NFC
<https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/nfc/push.html?l=73>,
Sensors
<https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/sensor/accelerometer.html?l=45>,
etc. So we think it would make sense to add a folder to include all of
these Test APIs in, straw-man proposal: platform-fakes.

../
../testharness.js
../testharnessreport.js
../platform-fakes/web-bluetooth-test.js
../platform-fakes/geolocation-test.js
....

Do y'all think this is a good approach?

Let me know what you think,

Gio