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/structured-attrs.sh

@@ -0,0 +1,23 @@
+source common.sh
+
+# 27ce722638 required some incompatible changes to the nix file, so skip this
+# tests for the older versions
+requireDaemonNewerThan "2.4pre20210712"
+
+clearStore
+
+rm -f $TEST_ROOT/result
+
+nix-build structured-attrs.nix -A all -o $TEST_ROOT/result
+
+[[ $(cat $TEST_ROOT/result/foo) = bar ]]
+[[ $(cat $TEST_ROOT/result-dev/foo) = foo ]]
+
+export NIX_BUILD_SHELL=$SHELL
+env NIX_PATH=nixpkgs=shell.nix nix-shell structured-attrs-shell.nix \
+    --run 'test -e .attrs.json; test "3" = "$(jq ".my.list|length" < $NIX_ATTRS_JSON_FILE)"'
+
+# `nix develop` is a slightly special way of dealing with environment vars, it parses
+# these from a shell-file exported from a derivation. This is to test especially `outputs`
+# (which is an associative array in thsi case) being fine.
+nix develop -f structured-attrs-shell.nix -c bash -c 'test -n "$out"'