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/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/functional/post-hook.sh b/tests/functional/post-hook.sh
new file mode 100644
index 000000000..752f8220c
--- /dev/null
+++ b/tests/functional/post-hook.sh
@@ -0,0 +1,31 @@
+source common.sh
+
+clearStore
+
+rm -f $TEST_ROOT/result
+
+export REMOTE_STORE=file:$TEST_ROOT/remote_store
+echo 'require-sigs = false' >> $NIX_CONF_DIR/nix.conf
+
+restartDaemon
+
+if isDaemonNewer "2.13"; then
+    pushToStore="$PWD/push-to-store.sh"
+else
+    pushToStore="$PWD/push-to-store-old.sh"
+fi
+
+# Build the dependencies and push them to the remote store.
+nix-build -o $TEST_ROOT/result dependencies.nix --post-build-hook "$pushToStore"
+# See if all outputs are passed to the post-build hook by only specifying one
+# We're not able to test CA tests this way
+export BUILD_HOOK_ONLY_OUT_PATHS=$([ ! $NIX_TESTS_CA_BY_DEFAULT ])
+nix-build -o $TEST_ROOT/result-mult multiple-outputs.nix -A a.first --post-build-hook "$pushToStore"
+
+clearStore
+
+# Ensure that the remote store contains both the runtime and build-time
+# closure of what we've just built.
+nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix
+nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix input1_drv
+nix copy --from "$REMOTE_STORE" --no-require-sigs -f multiple-outputs.nix a^second