Run integration tests in Travis

Travis now offers /dev/kvm, and its machines are powerful enough to run
our browser integration tests, at least for small projects.

Building an RPM on the Ubuntu host environment is a bit tricky, as there
are no installed RPMs. Thus ignore the BuildRequires, and install
appstream-util explicitly. In the future, the rpm/deb build should
happen inside the VM (like Cockpit does).

Add two scenarios for current Fedora and CentOS 8.

Switch to the "minimal" environment to make the test easier to reproduce
locally and more explicit.

Closes #386

.travis.yml

@@ -1,12 +1,31 @@
 dist: focal
-sudo: false
-language: node_js
-node_js:
-  - "lts/*"
+sudo: true
+language: minimal
 addons:
   apt:
     packages:
+      - appstream-util
+      - chromium-browser
+      - curl
+      - git
+      - libvirt-daemon-system
+      - npm
+      - python3-libvirt
+      - qemu-kvm
+      - qemu-utils
+      - rpm
       - sassc
+env:
+  - TEST_OS=fedora-33
+  - TEST_OS=centos-8-stream
 script:
-  - make
+  # HACK: /dev/kvm is root:kvm 0660 by default
+  - sudo chmod 666 /dev/kvm
+
+  # test PO template generation
   - make po/starter-kit.pot
+
+  # FIXME: build rpm inside VM; no installed rpms on Travis Ubuntu environment
+  - sed -i '/^BuildRequires:/d' *.spec.in
+
+  - TEST_JOBS=$(nproc) make check