Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

(cherry picked from commit 68c81c7375)

tests/functional/common.sh

@@ -0,0 +1,12 @@
+set -eu -o pipefail
+
+if [[ -z "${COMMON_SH_SOURCED-}" ]]; then
+
+COMMON_SH_SOURCED=1
+
+source "$(readlink -f "$(dirname "${BASH_SOURCE[0]-$0}")")/common/vars-and-functions.sh"
+if [[ -n "${NIX_DAEMON_PACKAGE:-}" ]]; then
+    startDaemon
+fi
+
+fi # COMMON_SH_SOURCED