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/flakes/search-root.sh

@@ -0,0 +1,50 @@
+source common.sh
+
+clearStore
+
+writeSimpleFlake $TEST_HOME
+cd $TEST_HOME
+mkdir -p foo/subdir
+
+echo '{ outputs = _: {}; }' > foo/flake.nix
+cat <<EOF > flake.nix
+{
+    inputs.foo.url = "$PWD/foo";
+    outputs = a: {
+       packages.$system = rec {
+         test = import ./simple.nix;
+         default = test;
+       };
+    };
+}
+EOF
+mkdir subdir
+pushd subdir
+
+success=("" . .# .#test ../subdir ../subdir#test "$PWD")
+failure=("path:$PWD")
+
+for i in "${success[@]}"; do
+    nix build $i || fail "flake should be found by searching up directories"
+done
+
+for i in "${failure[@]}"; do
+    ! nix build $i || fail "flake should not search up directories when using 'path:'"
+done
+
+popd
+
+nix build --override-input foo . || fail "flake should search up directories when not an installable"
+
+sed "s,$PWD/foo,$PWD/foo/subdir,g" -i flake.nix
+! nix build || fail "flake should not search upwards when part of inputs"
+
+if [[ -n $(type -p git) ]]; then
+    pushd subdir
+    git init
+    for i in "${success[@]}" "${failure[@]}"; do
+        ! nix build $i || fail "flake should not search past a git repository"
+    done
+    rm -rf .git
+    popd
+fi