tests: Support custom konan.home for platform lib tests

* Move testLibrary tasks from klib into backend.native/tests
 * Support konan.home property in them

backend.native/tests/testLibrary/kotlin/test_platform_lib.kt

@@ -0,0 +1,9 @@
+package test.konan.platform
+
+fun produceMessage() {
+    println("""This is a side effect of a test library linked into the binary.
+You should not be seeing this.
+""")
+}
+
+val x: Unit = produceMessage()