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, 58 insertions, 0 deletions

diff --git a/tests/functional/fixed.nix b/tests/functional/fixed.nix
new file mode 100644
index 000000000..babe71504
--- /dev/null
+++ b/tests/functional/fixed.nix
@@ -0,0 +1,58 @@
+with import ./config.nix;
+
+rec {
+
+  f2 = dummy: builder: mode: algo: hash: mkDerivation {
+    name = "fixed";
+    inherit builder;
+    outputHashMode = mode;
+    outputHashAlgo = algo;
+    outputHash = hash;
+    inherit dummy;
+    impureEnvVars = ["IMPURE_VAR1" "IMPURE_VAR2"];
+  };
+
+  f = f2 "";
+
+  good = [
+    (f ./fixed.builder1.sh "flat" "md5" "8ddd8be4b179a529afa5f2ffae4b9858")
+    (f ./fixed.builder1.sh "flat" "sha1" "a0b65939670bc2c010f4d5d6a0b3e4e4590fb92b")
+    (f ./fixed.builder2.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+    (f ./fixed.builder2.sh "recursive" "sha1" "vw46m23bizj4n8afrc0fj19wrp7mj3c0")
+  ];
+
+  # Expression to test that `nix-build --check` also throws an error if the hash of
+  # fixed-output derivation has changed even if the hash exists in the store (in this
+  # case the hash exists because of `fixed.builder2.sh`, but building a derivation
+  # with the same hash and a different result must throw an error).
+  check = [
+    (f ./fixed.builder1.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+  ];
+
+  good2 = [
+    # Yes, this looks fscked up: builder2 doesn't have that result.
+    # But Nix sees that an output with the desired hash already
+    # exists, and will refrain from building it.
+    (f ./fixed.builder2.sh "flat" "md5" "8ddd8be4b179a529afa5f2ffae4b9858")
+  ];
+
+  sameAsAdd =
+    f ./fixed.builder2.sh "recursive" "sha256" "1ixr6yd3297ciyp9im522dfxpqbkhcw0pylkb2aab915278fqaik";
+
+  bad = [
+    (f ./fixed.builder1.sh "flat" "md5" "0ddd8be4b179a529afa5f2ffae4b9858")
+  ];
+
+  reallyBad = [
+    # Hash too short, and not base-32 either.
+    (f ./fixed.builder1.sh "flat" "md5" "ddd8be4b179a529afa5f2ffae4b9858")
+  ];
+
+  # Test for building two derivations in parallel that produce the
+  # same output path because they're fixed-output derivations.
+  parallelSame = [
+    (f2 "foo" ./fixed.builder2.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+    (f2 "bar" ./fixed.builder2.sh "recursive" "md5" "3670af73070fa14077ad74e0f5ea4e42")
+  ];
+
+}