[FIX] Move backing field bytecodeListing tests to a separate folder

compiler/fir/analysis-tests/testData/resolveWithStdlib/properties/explicitBackingFieldUnsupported.fir.txt

@@ -0,0 +1,4 @@
+FILE: explicitBackingFieldUnsupported.kt
+    public final val that: R|kotlin/Number|
+        private <explicit backing field>: R|kotlin/Int| = Int(4)
+        public get(): R|kotlin/Number|

compiler/fir/analysis-tests/testData/resolveWithStdlib/properties/explicitBackingFieldUnsupported.kt

@@ -0,0 +1,4 @@
+// LANGUAGE: -ExplicitBackingFields
+
+val that: Number
+    <!UNSUPPORTED_FEATURE!>field = 4<!>