Build: Enable testing without magic comments (backports #46180)

[nik9000]

Previously we only turned on tests if we saw either // CONSOLE or
// TEST. These magic comments are difficult for the docs build to deal
with so it has moved away from using them where possible. We should
catch up. This adds another trigger to enable testing: marking a snippet
with the console language. It looks like this:

[source,console]
----
GET /
----

This saves a line which is nice, I guess. But it is more important to me
that this is consistent with the way the docs build works now.

Similarly this enables response testing when you mark a snippet with the
language console-result. That looks like:

[source,console-result]
----
{
  "result": "0.1"
}
----

// TESTRESPONSE is still available for situations like // TEST: when
the response isn't in the console-result language (like _cat) or
when you want to perform substitutions on the generated test.

Should unblock #46159.

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