Fix test data for -Xuse-mixed-named-arguments

Remove incorrect jar clash warning and avoid unrelated unused parameter
warnings

compiler/testData/cli/jvm/useMixedNamedArgumentsFlag.kt

@@ -1,4 +1,4 @@
-fun foo(x: Int, y: Int, z: Int) {}
+fun foo(x: Int, y: Int, z: Int): Int = x + y + z
 
 fun test() {
     foo(1, y = 2, 3)

compiler/testData/cli/jvm/useMixedNamedArgumentsFlag.out

@@ -1,12 +1 @@
-warning: some JAR files in the classpath have the Kotlin Runtime library bundled into them. This may cause difficult to debug problems if there's a different version of the Kotlin Runtime library in the classpath. Consider removing these libraries from the classpath
-dist/kotlinc/lib/kotlin-reflect.jar: warning: library has Kotlin runtime bundled into it
-compiler/testData/cli/jvm/useMixedNamedArgumentsFlag.kt:1:9: warning: parameter 'x' is never used
-fun foo(x: Int, y: Int, z: Int) {}
-        ^
-compiler/testData/cli/jvm/useMixedNamedArgumentsFlag.kt:1:17: warning: parameter 'y' is never used
-fun foo(x: Int, y: Int, z: Int) {}
-                ^
-compiler/testData/cli/jvm/useMixedNamedArgumentsFlag.kt:1:25: warning: parameter 'z' is never used
-fun foo(x: Int, y: Int, z: Int) {}
-                        ^
 OK