JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/kt313.kt

@@ -0,0 +1,11 @@
+// KT-313 Bug in substitutions in a function returning its type parameter T
+
+fun <T> Iterable<T>.join(separator : String?) : String {
+    return separator.npe()
+}
+
+fun <T : Any> T?.npe() : T {
+    if (this == null)
+      throw NullPointerException()
+    return this;
+}