JetDiagnosticsTest migrated to TestGenerator

- test data files renamed from *.jet to *.kt

compiler/testData/diagnostics/tests/MergePackagesWithJava.kt

@@ -0,0 +1,8 @@
+// KT-689 Allow to put Java and Kotlin files in the same packages
+
+// This is a stub test. One should not extend Java packages that come from libraries.
+package java
+
+val c : lang.Class<*>? = null
+
+val <T> Array<T>?.length : Int get() = if (this != null) this.size else throw NullPointerException()