gitignore: ignore framework/qml/build-tests/

The make qmltest target writes its CMake build tree to build-tests/
(deliberately distinct from the regular build/ tree so a configured-with
-DBUILD_TESTING=ON tree doesn't shadow production builds). The existing
build/ patterns don't match it, so it kept showing up as untracked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,8 @@
 # Build artefacts
 build/
 **/build/
+build-tests/
+**/build-tests/
 
 # Composer
 vendor/