[Native][tests] Introduce NATIVE_STANDALONE directive

This directive makes the test standalone, i.e. disables grouping it with
other tests into a single compilation, and in particular disables
package renaming for the test.

Also, remove the old hard-coded way to mark tests standalone.

compiler/testData/codegen/box/annotations/instances/annotationToString.kt

@@ -2,6 +2,11 @@
 // IGNORE_BACKEND: WASM
 // DONT_TARGET_EXACT_BACKEND: JS
 
+// This test fails on Native with test grouping and package renaming enabled,
+// because the latter doesn't yet handle annotation toString implementations properly.
+// Disable test grouping as a workaround:
+// NATIVE_STANDALONE
+
 // WITH_STDLIB
 // !LANGUAGE: +InstantiationOfAnnotationClasses