Testsuite: Run UIs in Qemu and check running processes and more (#982)

* Testsuite: Run UIs in Qemu and check running processes (and other changes)

* When `pmbootstrap qemu` gets killed, it now takes down the Qemu process with it
* `test/check_checksums.py` got a new optional `--build` parameter, which makes
  it build all changed packages instead of just checking the checksums
* We run this before running the testsuite now, so all changed packages get
  built before running tests (otherwise tests would hang without any output
  while a changed package is building)
* New testcase, that zaps all chroots, installs a specific UI (xfce4 and
  plasma-mobile currently, easy to extend), runs it via Qemu and checks the
  running processes via SSH.
* Version checking testcase: rewritten to include Alpine's testsuite file in
  our source tree, so we don't need to clone their git repo anymore. Now it
  is enabled for Travis.
* All this gives us a nice 10% code coverage boost
* Increased the `hello-world` pkgrel to verify that the Travis job is working.

* Various fixes
* Build device-packages for the device arch and don't raise an
  exception, but print a note if --ignore-depends is not specified
  and therefore the kernel gets installed, too.
* Don't use --force when building in Travis (because abuild doesn't
  check the checksums then. Bug report on the way.)
* Don't run the building process in the background, but wait for its
  completion
* Exit with 1 when showing usage in check_checksums.py

main/hello-world/APKBUILD

@@ -1,6 +1,6 @@
 pkgname=hello-world
 pkgver=1
-pkgrel=3
+pkgrel=4
 pkgdesc="hello world program to be built in the testsuite"
 url="https://en.wikipedia.org/wiki/%22Hello,_World!%22_program"
 arch="all"