tests: Merge installed/ and fedora-str/ directories

Let's be opinionated now, and our installed/ test story *is*
Ansible/STR.  Merge `tests/fedora-str` into `tests/installed/`.

Rework the nondestructive tests into a separate playbook run, and parallelize
them for more efficiency.

The destructive tests are also changed to use Ansible more.

Add a higher level `run.sh` entrypoint and update the `README.md`
with some useful tips.

Closes: #1513
Approved by: jlebon

ci/fah27-insttests.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/bash
+set -xeuo pipefail
+
+./tests/installed/provision.sh
+# TODO: enhance papr to have caching, a bit like https://docs.travis-ci.com/user/caching/
+cd tests/installed
+curl -Lo fedora-atomic-host.qcow2 https://getfedora.org/atomic_qcow2_latest
+exec env "TEST_SUBJECTS=$(pwd)/fedora-atomic-host.qcow2" ./run.sh