Remove obsolete Firefox preferences for testing

The following preferences don't have to be set manually anymore:

- `print.show_print_progress`
  - Why: removed
  - Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1770536
  - Commit: b522664310

- `gfx.offscreencanvas.enabled`
  - Why: removed
  - Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1914083
  - Commit: 7d2c688920

- `layout.css.round.enabled`
  - Why: removed
  - Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1874633
  - Commit: 97909653e7

- `dom.events.asyncClipboard.clipboardItem`
  - Why: enabled by default
  - Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1887845
  - Commit: de56ae88bf

test/test.mjs

@@ -942,18 +942,11 @@ async function startBrowser({
       "browser.download.dir": tempDir,
       // Print silently in a pdf
       "print.always_print_silent": true,
-      "print.show_print_progress": false,
       print_printer: "PDF",
       "print.printer_PDF.print_to_file": true,
       "print.printer_PDF.print_to_filename": printFile,
-      // Enable OffscreenCanvas
-      "gfx.offscreencanvas.enabled": true,
       // Disable gpu acceleration
       "gfx.canvas.accelerated": false,
-      // Enable the `round` CSS function.
-      "layout.css.round.enabled": true,
-      // This allow to copy some data in the clipboard.
-      "dom.events.asyncClipboard.clipboardItem": true,
       // It's helpful to see where the caret is.
       "accessibility.browsewithcaret": true,
       // Disable the newtabpage stuff.