[FIR] KT-59421 add checker context receiver property backing field

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

@@ -1,17 +0,0 @@
-// !LANGUAGE: +ContextReceivers
-
-interface A {
-    fun a(): Int
-}
-interface B {
-    fun b(): Int
-}
-
-context(A)
-val a = 1
-
-context(A, B)
-var b = 2
-
-context(A, B)
-val c get() = a() + b()

compiler/testData/diagnostics/tests/extensions/contextReceivers/noBackingField.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: +ContextReceivers
 
 interface A {