JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/regressions/Jet69.kt

@@ -0,0 +1,10 @@
+class Command() {}
+
+fun parse(<!UNUSED_PARAMETER!>cmd<!>: String): Command? { return null  }
+
+fun Any.equals(other : Any?) : Boolean = this === other
+
+fun main(args: Array<String>) {
+    val command = parse("")
+    if (command == null) 1 // error on this line, but must be OK
+}