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/ca/build-with-garbage-path.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Regression test for https://github.com/NixOS/nix/issues/4858
+
+source common.sh
+
+requireDaemonNewerThan "2.4pre20210621"
+
+# Get the output path of `rootCA`, and put some garbage instead
+outPath="$(nix-build ./content-addressed.nix -A rootCA --no-out-link)"
+nix-store --delete $(nix-store -q --referrers-closure "$outPath")
+touch "$outPath"
+
+# The build should correctly remove the garbage and put the expected path instead
+nix-build ./content-addressed.nix -A rootCA --no-out-link
+
+# Rebuild it. This shouldn’t overwrite the existing path
+oldInode=$(stat -c '%i' "$outPath")
+nix-build ./content-addressed.nix -A rootCA --no-out-link --arg seed 2
+newInode=$(stat -c '%i' "$outPath")
+[[ "$oldInode" == "$newInode" ]]