Native: correct ObjCExport test for forward declarations

The test relies on the fact that any name can be imported from a "magic"
package like `objcnames.classes`.
This was the case for K1, but likely won't be for K2: the compiler
should allow importing only those names that are actually
forward-declared in a cinterop library.

This commit changes the test that way. In particular, moves it to a
different task that has a cinterop dependency.

kotlin-native/backend.native/tests/framework/forwardDeclarations/lib.kt

@@ -0,0 +1,9 @@
+import clib.*
+import cnames.structs.ForwardDeclaredStruct
+import objcnames.classes.ForwardDeclaredClass
+import objcnames.protocols.ForwardDeclaredProtocolProtocol
+import kotlinx.cinterop.CPointer
+
+fun sameForwardDeclaredStruct(ptr: CPointer<ForwardDeclaredStruct>?): CPointer<ForwardDeclaredStruct>? = sameStruct(ptr)
+fun sameForwardDeclaredClass(obj: ForwardDeclaredClass?): ForwardDeclaredClass? = sameClass(obj)
+fun sameForwardDeclaredProtocol(obj: ForwardDeclaredProtocolProtocol?): ForwardDeclaredProtocolProtocol? = sameProtocol(obj)