Document how to set the backend for the tests.

I only had the old way save in my bash history and this one was only
mentioned in the changelog.

Also changed the heading above the new entry to be title case, which
seems to be more consistent with the other headinfs in the file.

Also remove the one remaining mention of `QUTE_BDD_WEBENGINE` since it
does nothing anymore.

.github/workflows/ci.yml

@@ -92,7 +92,6 @@ jobs:
     container:
       image: "qutebrowser/ci:${{ matrix.image }}"
       env:
-        QUTE_BDD_WEBENGINE: "${{ matrix.image != 'archlinux-webkit' }}"
         DOCKER: "${{ matrix.image }}"
         CI: true
         PYTEST_ADDOPTS: "--color=yes"

doc/contributing.asciidoc

@@ -153,7 +153,7 @@ smallest scope which makes sense. Most of the time, this will be line scope.
 false-positives, let me know! I'm still tweaking the parameters.
 
 
-Running Specific Tests
+Running specific tests
 ~~~~~~~~~~~~~~~~~~~~~~
 
 While you are developing you often don't want to run the full test
@@ -180,6 +180,15 @@ tox -e py37 -- tests/end2end/features/test_tabs_bdd.py -k undo
 tox -e py37-cov -- tests/unit/browser/test_webelem.py
 ----
 
+Specifying the backend for tests
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Tests automatically pick the backend based on what they manage to import. If
+you have both backends available and you would like the tests to be run with a
+specific one you can set either of a) the environment variable QUTE_TESTS_BACKEND
+, or b) the command line argument --qute-backend, to the desired backend
+(webkit/webengine).
+
 Profiling
 ~~~~~~~~~