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

idea/testData/quickfix/expressions/removeUselessCastUnderSmartCast.kt

@@ -0,0 +1,7 @@
+// "Remove cast" "true"
+fun test(x: Any): String? {
+    if (x is String) {
+        return x <caret>as String
+    }
+    return null
+}