Add some integration tests for the CLI’s quiet mode

See gh-6918

spring-boot-cli/src/it/resources/run-command/quiet.groovy

@@ -0,0 +1,10 @@
+package org.test
+
+@Component
+class Example implements CommandLineRunner {
+
+	void run(String... args) {
+		print "Ssshh"
+	}
+
+}