Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/reflection/callBy/extensionFunction.kt

@@ -0,0 +1,11 @@
+// WITH_REFLECT
+
+import kotlin.test.assertEquals
+
+fun String.sum(other: String = "b") = this + other
+
+fun box(): String {
+    val f = String::sum
+    assertEquals("ab", f.callBy(mapOf(f.parameters.first() to "a")))
+    return "OK"
+}