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/lang/eval-fail-foldlStrict-strict-op-application.err.exp

@@ -0,0 +1,38 @@
+error:
+       … while calling the 'foldl'' builtin
+
+         at /pwd/lang/eval-fail-foldlStrict-strict-op-application.nix:2:1:
+
+            1| # Tests that the result of applying op is forced even if the value is never used
+            2| builtins.foldl'
+             | ^
+            3|   (_: f: f null)
+
+       … while calling anonymous lambda
+
+         at /pwd/lang/eval-fail-foldlStrict-strict-op-application.nix:3:7:
+
+            2| builtins.foldl'
+            3|   (_: f: f null)
+             |       ^
+            4|   null
+
+       … from call site
+
+         at /pwd/lang/eval-fail-foldlStrict-strict-op-application.nix:3:10:
+
+            2| builtins.foldl'
+            3|   (_: f: f null)
+             |          ^
+            4|   null
+
+       … while calling anonymous lambda
+
+         at /pwd/lang/eval-fail-foldlStrict-strict-op-application.nix:5:6:
+
+            4|   null
+            5|   [ (_: throw "Not the final value, but is still forced!") (_: 23) ]
+             |      ^
+            6|
+
+       error: Not the final value, but is still forced!