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, 29 insertions, 0 deletions

diff --git a/tests/functional/hash-check.nix b/tests/functional/hash-check.nix
new file mode 100644
index 000000000..4a8e9b8a8
--- /dev/null
+++ b/tests/functional/hash-check.nix
@@ -0,0 +1,29 @@
+let {
+
+  input1 = derivation {
+    name = "dependencies-input-1";
+    system = "i086-msdos";
+    builder = "/bar/sh";
+    args = ["-e" "-x" ./dummy];
+  };
+
+  input2 = derivation {
+    name = "dependencies-input-2";
+    system = "i086-msdos";
+    builder = "/bar/sh";
+    args = ["-e" "-x" ./dummy];
+    outputHashMode = "recursive";
+    outputHashAlgo = "md5";
+    outputHash = "ffffffffffffffffffffffffffffffff";
+  };
+
+  body = derivation {
+    name = "dependencies";
+    system = "i086-msdos";
+    builder = "/bar/sh";
+    args = ["-e" "-x" (./dummy  + "/FOOBAR/../.")];
+    input1 = input1 + "/.";
+    inherit input2;
+  };
+
+}
\ No newline at end of file