test(firefox): introduce vendor-specific specs (#3890)

Certain Puppeteer methods do expose the inner browser - e.g.
`browser.version()` depends on the browser we run.

Split out these tests into a vendor-specific test suites.

References #3889

.gitignore

@@ -1,5 +1,6 @@
 /node_modules/
-/test/output
+/test/output-chromium
+/test/output-firefox
 /test/test-user-data-dir*
 /.local-chromium/
 /.dev_profile*