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/dyn-drv/recursive-mod-json.sh

@@ -1,27 +0,0 @@
-source common.sh
-
-# FIXME
-if [[ $(uname) != Linux ]]; then skipTest "Not running Linux"; fi
-
-export NIX_TESTS_CA_BY_DEFAULT=1
-
-enableFeatures 'recursive-nix'
-restartDaemon
-
-clearStore
-
-rm -f $TEST_ROOT/result
-
-EXTRA_PATH=$(dirname $(type -p nix)):$(dirname $(type -p jq))
-export EXTRA_PATH
-
-# Will produce a drv
-metaDrv=$(nix-instantiate ./recursive-mod-json.nix)
-
-# computed "dynamic" derivation
-drv=$(nix-store -r $metaDrv)
-
-# build that dyn drv
-res=$(nix-store -r $drv)
-
-grep 'I am alive!' $res/hello