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/remote-store.sh

@@ -0,0 +1,43 @@
+source common.sh
+
+clearStore
+
+# Ensure "fake ssh" remote store works just as legacy fake ssh would.
+nix --store ssh-ng://localhost?remote-store=$TEST_ROOT/other-store doctor
+
+# Ensure that store ping trusted works with ssh-ng://
+nix --store ssh-ng://localhost?remote-store=$TEST_ROOT/other-store store ping --json | jq -e '.trusted'
+
+startDaemon
+
+if isDaemonNewer "2.15pre0"; then
+    # Ensure that ping works trusted with new daemon
+    nix store ping --json | jq -e '.trusted'
+else
+    # And the the field is absent with the old daemon
+    nix store ping --json | jq -e 'has("trusted") | not'
+fi
+
+# Test import-from-derivation through the daemon.
+[[ $(nix eval --impure --raw --expr '
+  with import ./config.nix;
+  import (
+    mkDerivation {
+      name = "foo";
+      bla = import ./dependencies.nix {};
+      buildCommand = "
+        echo \\\"hi\\\" > $out
+      ";
+    }
+  )
+') = hi ]]
+
+storeCleared=1 NIX_REMOTE_=$NIX_REMOTE $SHELL ./user-envs.sh
+
+nix-store --gc --max-freed 1K
+
+nix-store --dump-db > $TEST_ROOT/d1
+NIX_REMOTE= nix-store --dump-db > $TEST_ROOT/d2
+cmp $TEST_ROOT/d1 $TEST_ROOT/d2
+
+killDaemon