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

idea/testData/quickfix/modifiers/nestedClassNotAllowed.kt

@@ -0,0 +1,7 @@
+// "Add 'inner' modifier" "true"
+class A() {
+    inner class B() {
+        class <caret>C() {
+        }
+    }
+}