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/build-remote-input-addressed.sh

@@ -1,33 +0,0 @@
-source common.sh
-
-file=build-hook.nix
-
-source build-remote.sh
-
-# Add a `post-build-hook` option to the nix conf.
-# This hook will be executed both for the local machine and the remote builders
-# (because they share the same config).
-registerBuildHook () {
-    # Dummy post-build-hook just to ensure that it's executed correctly.
-    # (we can't reuse the one from `$PWD/push-to-store.sh` because of
-    # https://github.com/NixOS/nix/issues/4341)
-    cat <<EOF > $TEST_ROOT/post-build-hook.sh
-#!/bin/sh
-
-echo "Post hook ran successfully"
-# Add an empty line to a counter file, just to check that this hook ran properly
-echo "" >> $TEST_ROOT/post-hook-counter
-EOF
-    chmod +x $TEST_ROOT/post-build-hook.sh
-    rm -f $TEST_ROOT/post-hook-counter
-
-    echo "post-build-hook = $TEST_ROOT/post-build-hook.sh" >> $NIX_CONF_DIR/nix.conf
-}
-
-registerBuildHook
-source build-remote.sh
-
-# `build-hook.nix` has four derivations to build, and the hook runs twice for
-# each derivation (once on the builder and once on the host), so the counter
-# should contain eight lines now
-[[ $(cat $TEST_ROOT/post-hook-counter | wc -l) -eq 8 ]]