Minor, refactor diagnostic tests on repeatable annotations

compiler/testData/diagnostics/tests/annotations/repeatable/javaUnrepeatable.fir.kt

@@ -0,0 +1,32 @@
+// FULL_JDK
+// FILE: Runtime.java
+
+import java.lang.annotation.*;
+
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Runtime {}
+
+// FILE: Clazz.java
+
+import java.lang.annotation.*;
+
+@Retention(RetentionPolicy.CLASS)
+public @interface Clazz {}
+
+// FILE: Source.java
+
+import java.lang.annotation.*;
+
+@Retention(RetentionPolicy.SOURCE)
+public @interface Source {}
+
+// FILE: usage.kt
+
+@Runtime @Runtime
+class UseRuntime
+
+@Clazz @Clazz
+class UseClazz
+
+@Source @Source
+class UseSource