Remove the streamqueue dependency

The `streamqueue` dependency is only used for the test targets in the
Gulpfile to make sure that the test types are run in series. This is
done by modelling the test processes as readable streams and then having
`streamqueue` combine them into a single readable stream for Gulp that
processes the inner readable streams in series (in contrast to the
`ordered-read-streams` dependency which is very similar but processes
the inner streams in parallel).

However, modelling the test processes as readable streams is a bit odd
because we're not actually streaming any data as one might expect.
Instead, we only use them to signal test process completion/abortion.

Fortunately nowadays, with modern Gulp versions, we don't need readable
streams and `streamqueue` anymore because we can achieve the same result
with simple asynchronous functions that can be passed to e.g.
`gulp.series()` calls. Note that we already do this in various places,
and overall it should be a better fit for test process invocations.

package.json

@@ -51,7 +51,6 @@
     "postcss-nesting": "^12.1.5",
     "prettier": "^3.3.3",
     "puppeteer": "^22.13.0",
-    "streamqueue": "^1.1.2",
     "stylelint": "^16.7.0",
     "stylelint-prettier": "^5.0.0",
     "terser-webpack-plugin": "^5.3.10",