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/ca/racy.nix

@@ -0,0 +1,15 @@
+# A derivation that would certainly fail if several builders tried to
+# build it at once.
+
+
+with import ./config.nix;
+
+mkDerivation {
+  name = "simple";
+  buildCommand = ''
+    mkdir $out
+    echo bar >> $out/foo
+    sleep 3
+    [[ "$(cat $out/foo)" == bar ]]
+  '';
+}