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, 13 insertions, 0 deletions

diff --git a/tests/functional/big-derivation-attr.nix b/tests/functional/big-derivation-attr.nix
new file mode 100644
index 000000000..35c1187f6
--- /dev/null
+++ b/tests/functional/big-derivation-attr.nix
@@ -0,0 +1,13 @@
+let
+  sixteenBytes = "0123456789abcdef";
+  times16 = s: builtins.concatStringsSep "" [s s s s s s s s s s s s s s s s];
+  exp = n: x: if n == 1 then x else times16 (exp (n - 1) x);
+  sixteenMegabyte = exp 6 sixteenBytes;
+in
+assert builtins.stringLength sixteenMegabyte == 16777216;
+derivation {
+  name = "big-derivation-attr";
+  builder = "/x";
+  system = "y";
+  bigAttr = sixteenMegabyte;
+}