These specs are only run on bundler 2, so the compatibility method is
not needed.

7161: Fix file URIs on Windows r=hsbt a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that file:// URIs were not properly handled on Windows, and those are heavily used by our specs.

### What was your diagnosis of the problem?

My diagnosis was that Windows paths do not start with a slash, so we need to add an extra slash to
separate the host from the path in file:// urls. Otherwise "D:" is parsed as the host segment in the URI.

As a consequence of that change, we also need to strip that leading character when using the path segment for those URIs, so that it's a valid file path on Windows.

This change reduces Windows CI spec failures from 429 to 355.

In order to apply this fix for all file URI's in tests, I extracted a helper method and searched and replaced all file:// ocurrences with the method. The diff is big because of that, but I think it works fine.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

* Drop bundler 1 stuff from tests.
* Move all feature flags to bundler 3 (like they are in 2-0-stable) and
get them tested.