JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/j+k/UnboxingNulls.kt

@@ -0,0 +1,13 @@
+// FILE: a/Test.java
+package a;
+
+public class Test<T> {
+  T t() {return null;}
+}
+// FILE: b.kt
+package a
+
+fun foo() {
+  // If this fails, it means that we have broken the rule that Java returns are always nullable
+  a.Test<Int>().t() <!UNSAFE_INFIX_CALL!>+<!> 1
+}