JetDiagnosticsTest migrated to TestGenerator

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

compiler/testData/diagnostics/tests/operatorsOverloading/AssignOperatorAmbiguity.kt

@@ -0,0 +1,17 @@
+//KT-1820 Write test for ASSIGN_OPERATOR_AMBIGUITY
+package kt1820
+
+class MyInt(val i: Int) {
+    fun plus(m: MyInt) : MyInt = MyInt(m.i + i)
+}
+
+fun Any.plusAssign(<!UNUSED_PARAMETER!>a<!>: Any) {}
+
+fun test(m: MyInt) {
+    m <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> m
+
+    var i = 1
+    i <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> 34
+}
+
+