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/zstd.sh

@@ -0,0 +1,28 @@
+source common.sh
+
+clearStore
+clearCache
+
+cacheURI="file://$cacheDir?compression=zstd"
+
+outPath=$(nix-build dependencies.nix --no-out-link)
+
+nix copy --to $cacheURI $outPath
+
+HASH=$(nix hash path $outPath)
+
+clearStore
+clearCacheCache
+
+nix copy --from $cacheURI $outPath --no-check-sigs
+
+if ls $cacheDir/nar/*.zst &> /dev/null; then
+    echo "files do exist"
+else
+    echo "nars do not exist"
+    exit 1
+fi
+
+HASH2=$(nix hash path $outPath)
+
+[[ $HASH = $HASH2 ]]