[build-logic] junit-platform-launcher must be testImplementation not …

…testRuntimeOnly

build-logic/src/main/groovy/org.jxmpp.java-conventions.gradle

@@ -95,7 +95,7 @@ dependencies {
 	testRuntimeOnly "org.junit.jupiter:junit-jupiter-engine:$junitVersion"
 
 	// https://stackoverflow.com/a/77274251/194894
-	testRuntimeOnly "org.junit.platform:junit-platform-launcher:1.11.0"
+	testImplementation "org.junit.platform:junit-platform-launcher:1.11.0"
 
 	errorprone 'com.google.errorprone:error_prone_core:2.32.0'
 }