tests/flakes: check git+file:./${submodule} protocol

Relative, local git repo used to work (for submodules), but it
fails after 3e0129c.

This commit adds a test to prevent such failure in the future.

tests/functional/flakes/flakes.sh

@@ -106,6 +106,14 @@ nix build -o "$TEST_ROOT/result" "git+file://$flake1Dir#default"
 nix build -o "$TEST_ROOT/result" "$flake1Dir?ref=HEAD#default"
 nix build -o "$TEST_ROOT/result" "git+file://$flake1Dir?ref=HEAD#default"
 
+# Check that relative paths are allowed for git flakes.
+# This may change in the future once git submodule support is refined.
+# See: https://discourse.nixos.org/t/57783 and #9708.
+(
+  cd "$flake1Dir/.."
+  nix build -o "$TEST_ROOT/result" "git+file:./$(basename "$flake1Dir")"
+)
+
 # Check that store symlinks inside a flake are not interpreted as flakes.
 nix build -o "$flake1Dir/result" "git+file://$flake1Dir"
 nix path-info "$flake1Dir/result"