Fix language features in tests for gradual migration to 1.3 (part 2)

In 1.3, due to changes in language, testdata for some tests can be
different from 1.2

We want to simlultaneously test both versions, so instead of fixing
language version in such tests, we split them into two: one with fixed
1.2, another with fixed 1.3

compiler/testData/diagnostics/tests/backingField/FieldReassignment_before.kt

@@ -1,3 +1,5 @@
+// !LANGUAGE: -RestrictionOfValReassignmentViaBackingField
+
 val my: Int = 1
     get() {
         <!VAL_REASSIGNMENT_VIA_BACKING_FIELD!>field<!>++