[KLIB][tests] Reorganize "klib dump-metadata" tests

- Keep the test data under "native/native.tests/testData/klib/" dir
- Rename tests from "klib contents" to "klib dump-metadata"

native/native.tests/testData/klib/dump-metadata/builtinsSerializer/simple.kt

@@ -0,0 +1,11 @@
+package test
+
+class Class {
+    fun member() = null
+}
+
+fun function(int: Int, string: String = "default"): Class = Class()
+
+fun <T> T.extension(): T? = null
+
+val property: Unit = Unit