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 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 56 insertions, 0 deletions

diff --git a/tests/functional/recursive.nix b/tests/functional/recursive.nix
new file mode 100644
index 000000000..fa8cc04db
--- /dev/null
+++ b/tests/functional/recursive.nix
@@ -0,0 +1,56 @@
+with import ./config.nix;
+
+mkDerivation rec {
+  name = "recursive";
+  dummy = builtins.toFile "dummy" "bla bla";
+  SHELL = shell;
+
+  # Note: this is a string without context.
+  unreachable = builtins.getEnv "unreachable";
+
+  NIX_TESTS_CA_BY_DEFAULT = builtins.getEnv "NIX_TESTS_CA_BY_DEFAULT";
+
+  requiredSystemFeatures = [ "recursive-nix" ];
+
+  buildCommand = ''
+    mkdir $out
+    opts="--experimental-features nix-command ${if (NIX_TESTS_CA_BY_DEFAULT == "1") then "--extra-experimental-features ca-derivations" else ""}"
+
+    PATH=${builtins.getEnv "NIX_BIN_DIR"}:$PATH
+
+    # Check that we can query/build paths in our input closure.
+    nix $opts path-info $dummy
+    nix $opts build $dummy
+
+    # Make sure we cannot query/build paths not in out input closure.
+    [[ -e $unreachable ]]
+    (! nix $opts path-info $unreachable)
+    (! nix $opts build $unreachable)
+
+    # Add something to the store.
+    echo foobar > foobar
+    foobar=$(nix $opts store add-path ./foobar)
+
+    nix $opts path-info $foobar
+    nix $opts build $foobar
+
+    # Add it to our closure.
+    ln -s $foobar $out/foobar
+
+    [[ $(nix $opts path-info --all | wc -l) -eq 4 ]]
+
+    # Build a derivation.
+    nix $opts build -L --impure --expr '
+      with import ${./config.nix};
+      mkDerivation {
+        name = "inner1";
+        buildCommand = "echo $fnord blaat > $out";
+        fnord = builtins.toFile "fnord" "fnord";
+      }
+    '
+
+    [[ $(nix $opts path-info --json ./result) =~ fnord ]]
+
+    ln -s $(nix $opts path-info ./result) $out/inner1
+  '';
+}