[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/inference/genericAssignmentOperator.fir.kt

@@ -0,0 +1,9 @@
+class R<T>
+
+fun <T> f(): R<T> = R<T>()
+
+operator fun Int.plusAssign(y: R<Int>) {}
+
+fun box() {
+    <!INAPPLICABLE_CANDIDATE!>1 += f()<!>
+}