Big change in quickfix tests: no stupid test data format with "before" and "after" file prefixes

idea/testData/quickfix/variables/changeMutability/valReassignmentProperty.kt

@@ -0,0 +1,8 @@
+// "Make variable mutable" "true"
+class A() {
+    val a = 1
+
+    fun foo() {
+        <caret>a = 5
+    }
+}