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

idea/testData/quickfix/checkArguments/mixedNamedAndPositionalArgumentsUsedPositional.kt

@@ -0,0 +1,6 @@
+// "Add name to argument: 'b = "FOO"'" "true"
+fun f(a: String, x: Int, b: String) {}
+
+fun g() {
+    f("BAR", x = 10, <caret>"FOO")
+}