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/referrers.sh

@@ -0,0 +1,38 @@
+source common.sh
+
+needLocalStore "uses some low-level store manipulations that aren’t available through the daemon"
+
+clearStore
+
+max=500
+
+reference=$NIX_STORE_DIR/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-bla
+touch $reference
+(echo $reference && echo && echo 0) | nix-store --register-validity
+
+echo "making registration..."
+
+set +x
+for ((n = 0; n < $max; n++)); do
+    storePath=$NIX_STORE_DIR/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-$n
+    echo -n > $storePath
+    ref2=$NIX_STORE_DIR/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-$((n+1))
+    if test $((n+1)) = $max; then
+        ref2=$reference
+    fi
+    echo $storePath; echo; echo 2; echo $reference; echo $ref2
+done > $TEST_ROOT/reg_info
+set -x
+
+echo "registering..."
+
+nix-store --register-validity < $TEST_ROOT/reg_info
+
+echo "collecting garbage..."
+ln -sfn $reference "$NIX_STATE_DIR"/gcroots/ref
+nix-store --gc
+
+if [ -n "$(type -p sqlite3)" -a "$(sqlite3 $NIX_STATE_DIR/db/db.sqlite 'select count(*) from Refs')" -ne 0 ]; then
+    echo "referrers not cleaned up"
+    exit 1
+fi