improve phpstan ignore error notation

phpstan.neon

 parameters:
 	checkMissingIterableValueType: false
 	ignoreErrors:
-		# intentionally; used to test if assertions are enabled in Tools.php
-		- '#Variable \$assertsEnabled in isset\(\) always exists and is not nullable.#'
+		# we need to test if assertions are enabled
+		-
+			message: "#^Variable \\$assertsEnabled in isset\\(\\) always exists and is not nullable\\.$#"
+			count: 1
+			path: src/Tools.php