FIR: Fix test data after making LHS of assignment an expression

KT-54648

compiler/testData/diagnostics/tests/generics/varProjection/setterProjectedOutNoPlusAssign.fir.kt

@@ -0,0 +1,10 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
+interface Tr<T> {
+    var v: T
+}
+
+fun test(t: Tr<out String>) {
+    // resolved as t.v = t.v + null!!, where type of right operand is String,
+    // so TYPE_MISMATCH: String is not <: of Captured(out String)
+    <!ASSIGNMENT_TYPE_MISMATCH!>t.v += null!!<!>
+}