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/dyn-drv/local.mk

@@ -0,0 +1,15 @@
+dyn-drv-tests := \
+  $(d)/text-hashed-output.sh \
+  $(d)/recursive-mod-json.sh \
+  $(d)/build-built-drv.sh \
+  $(d)/eval-outputOf.sh \
+  $(d)/dep-built-drv.sh \
+  $(d)/old-daemon-error-hack.sh
+
+install-tests-groups += dyn-drv
+
+clean-files += \
+  $(d)/config.nix
+
+test-deps += \
+  tests/functional/dyn-drv/config.nix