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, 17 insertions, 0 deletions

diff --git a/tests/functional/store-ping.sh b/tests/functional/store-ping.sh
new file mode 100644
index 000000000..9846c7d3d
--- /dev/null
+++ b/tests/functional/store-ping.sh
@@ -0,0 +1,17 @@
+source common.sh
+
+STORE_INFO=$(nix store ping 2>&1)
+STORE_INFO_JSON=$(nix store ping --json)
+
+echo "$STORE_INFO" | grep "Store URL: ${NIX_REMOTE}"
+
+if [[ -v NIX_DAEMON_PACKAGE ]] && isDaemonNewer "2.7.0pre20220126"; then
+    DAEMON_VERSION=$($NIX_DAEMON_PACKAGE/bin/nix-daemon --version | cut -d' ' -f3)
+    echo "$STORE_INFO" | grep "Version: $DAEMON_VERSION"
+    [[ "$(echo "$STORE_INFO_JSON" | jq -r ".version")" == "$DAEMON_VERSION" ]]
+fi
+
+expect 127 NIX_REMOTE=unix:$PWD/store nix store ping || \
+    fail "nix store ping on a non-existent store should fail"
+
+[[ "$(echo "$STORE_INFO_JSON" | jq -r ".url")" == "${NIX_REMOTE:-local}" ]]