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/user-envs-migration.sh

@@ -0,0 +1,35 @@
+# Test that the migration of user environments
+# (https://github.com/NixOS/nix/pull/5226) does preserve everything
+
+source common.sh
+
+if isDaemonNewer "2.4pre20211005"; then
+    skipTest "Daemon is too new"
+fi
+
+
+killDaemon
+unset NIX_REMOTE
+
+clearStore
+clearProfiles
+rm -rf ~/.nix-profile
+
+# Fill the environment using the older Nix
+PATH_WITH_NEW_NIX="$PATH"
+export PATH="$NIX_DAEMON_PACKAGE/bin:$PATH"
+
+nix-env -f user-envs.nix -i foo-1.0
+nix-env -f user-envs.nix -i bar-0.1
+
+# Migrate to the new profile dir, and ensure that everything’s there
+export PATH="$PATH_WITH_NEW_NIX"
+nix-env -q # Trigger the migration
+( [[ -L ~/.nix-profile ]] && \
+    [[ $(readlink ~/.nix-profile) == ~/.local/share/nix/profiles/profile ]] ) || \
+    fail "The nix profile should point to the new location"
+
+(nix-env -q | grep foo && nix-env -q | grep bar && \
+    [[ -e ~/.nix-profile/bin/foo ]] && \
+    [[ $(nix-env --list-generations | wc -l) == 2 ]]) ||
+    fail "The nix profile should have the same content as before the migration"