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/flakes/show.sh

@@ -0,0 +1,87 @@
+source ./common.sh
+
+flakeDir=$TEST_ROOT/flake
+mkdir -p "$flakeDir"
+
+writeSimpleFlake "$flakeDir"
+cd "$flakeDir"
+
+
+# By default: Only show the packages content for the current system and no
+# legacyPackages at all
+nix flake show --json > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.packages.someOtherSystem.default == {};
+assert show_output.packages.${builtins.currentSystem}.default.name == "simple";
+assert show_output.legacyPackages.${builtins.currentSystem} == {};
+true
+'
+
+# With `--all-systems`, show the packages for all systems
+nix flake show --json --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.packages.someOtherSystem.default.name == "simple";
+assert show_output.legacyPackages.${builtins.currentSystem} == {};
+true
+'
+
+# With `--legacy`, show the legacy packages
+nix flake show --json --legacy > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.legacyPackages.${builtins.currentSystem}.hello.name == "simple";
+true
+'
+
+# Test that attributes are only reported when they have actual content
+cat >flake.nix <<EOF
+{
+  description = "Bla bla";
+
+  outputs = inputs: rec {
+    apps.$system = { };
+    checks.$system = { };
+    devShells.$system = { };
+    legacyPackages.$system = { };
+    packages.$system = { };
+    packages.someOtherSystem = { };
+
+    formatter = { };
+    nixosConfigurations = { };
+    nixosModules = { };
+  };
+}
+EOF
+nix flake show --json --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output == { };
+true
+'
+
+# Test that attributes with errors are handled correctly.
+# nixpkgs.legacyPackages is a particularly prominent instance of this.
+cat >flake.nix <<EOF
+{
+  outputs = inputs: {
+    legacyPackages.$system = {
+      AAAAAASomeThingsFailToEvaluate = throw "nooo";
+      simple = import ./simple.nix;
+    };
+  };
+}
+EOF
+nix flake show --json --legacy --all-systems > show-output.json
+nix eval --impure --expr '
+let show_output = builtins.fromJSON (builtins.readFile ./show-output.json);
+in
+assert show_output.legacyPackages.${builtins.currentSystem}.AAAAAASomeThingsFailToEvaluate == { };
+assert show_output.legacyPackages.${builtins.currentSystem}.simple.name == "simple";
+true
+'