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.nix

@@ -0,0 +1,28 @@
+with import ./config.nix;
+
+let
+
+  input = import ./simple.nix;
+
+  dependent = mkDerivation {
+    name = "dependent";
+    buildCommand = ''
+      mkdir -p $out
+      echo -n "$input1" > "$out/file1"
+      echo -n "$input2" > "$out/file2"
+    '';
+    input1 = "${input}/hello";
+    input2 = "hello";
+  };
+
+  readDependent = mkDerivation {
+    # Will evaluate correctly because file2 doesn't have any references,
+    # even though the `dependent` derivation does.
+    name = builtins.readFile (dependent + "/file2");
+    buildCommand = ''
+      echo "$input" > "$out"
+    '';
+    input = builtins.readFile (dependent + "/file1");
+  };
+
+in readDependent