JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/kt312.kt

@@ -0,0 +1,10 @@
+// KT-312 Nullability problem when a nullable version of a generic type is returned
+
+fun <T> Array<out T>.safeGet(index : Int) : T? {
+    return if (index < size) this[index] else null
+}
+
+val args : Array<String> = Array<String>(1, {""})
+val name : String = <!TYPE_MISMATCH!>args.safeGet<String>(0)<!> // No error, must be type mismatch
+val name1 : String? = args.safeGet(0)
+