FIR: Adjust diagnostics test data after resolution of context receivers

compiler/testData/diagnostics/tests/extensions/contextReceivers/thisIdentifierInfo.fir.kt

@@ -3,6 +3,6 @@
 class A(val a: String?)
 
 context(A) fun f() {
-    if (this<!UNRESOLVED_LABEL!>@A<!>.a == null) return
-    this<!UNRESOLVED_LABEL!>@A<!>.a.length
-}
+    if (this@A.a == null) return
+    this@A.a.length
+}