Test(TravisCI): run tests in Firefox (#5831)

* trying to run tests in FF

* run tests in PhantomJS first

* add chrome test run to travis

* Revert "add chrome test run to travis"

This reverts commit 933f7b5e038722b18b62180edfe4cf19aadc11ab.

* update Firefox test script in Travis CI

* use latest version of Firefox in TravisCI tests

* fix firefox artifact signature in travis config

* trying to set screen size with xvfb-run

* increase screen size

* remove DISPLAY env variable

.travis.yml

@@ -8,6 +8,7 @@ addons:
     - dist
     target_paths:
     - content/leaflet/${NAME}
+  firefox: latest
 env:
   global:
   - ARTIFACTS_BUCKET=leafletjs-cdn
@@ -22,6 +23,7 @@ env:
       fSJXkxmQ4MAjUdNFDIirBHhdZme8q3PueFzJ+5odFMvPGn/aITQ=
 script:
   - npm test
+  - xvfb-run --server-args="-screen 0 1280x1024x16" npm run test-nolint -- --browsers Firefox
 after_success:
   - npm run build
   - cd dist && zip -x .DS_Store -r leaflet.zip . && cd ..