test: refactor WPTRunner and enable parallel WPT execution

Author: panva

Makefile

@@ -592,7 +592,8 @@ test-wpt: all
 test-wpt-report:
 	$(RM) -r out/wpt
 	mkdir -p out/wpt
-	WPT_REPORT=1 $(PYTHON) tools/test.py --shell $(NODE) $(PARALLEL_ARGS) wpt
+	-WPT_REPORT=1 $(PYTHON) tools/test.py --shell $(NODE) $(PARALLEL_ARGS) wpt
+	$(NODE) "$$PWD/tools/merge-wpt-reports.mjs"
 
 .PHONY: test-internet
 test-internet: all