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/
```

tests/functional/readfile-context.sh

@@ -0,0 +1,16 @@
+source common.sh
+
+clearStore
+
+outPath=$(nix-build --no-out-link readfile-context.nix)
+
+# Set a GC root.
+ln -s $outPath "$NIX_STATE_DIR"/gcroots/foo
+
+# Check that file exists.
+[ "$(cat $(cat $outPath))" = "Hello World!" ]
+
+nix-collect-garbage
+
+# Check that file still exists.
+[ "$(cat $(cat $outPath))" = "Hello World!" ]