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/dyn-drv/old-daemon-error-hack.nix

@@ -0,0 +1,20 @@
+with import ./config.nix;
+
+# A simple content-addressed derivation.
+# The derivation can be arbitrarily modified by passing a different `seed`,
+# but the output will always be the same
+rec {
+  stub = mkDerivation {
+    name = "stub";
+    buildCommand = ''
+      echo stub > $out
+    '';
+  };
+  wrapper = mkDerivation {
+    name = "has-dynamic-drv-dep";
+    buildCommand = ''
+      exit 1 # we're not building this derivation
+      ${builtins.outputOf stub.outPath "out"}
+    '';
+  };
+}