feat: add va_list version of wrenCall and fix GC bug with return values

- Introduce wrenCallVarArgs as an explicit va_list variant, allowing variadic functions to forward their arguments directly without re-parsing.
- Fix a garbage collection issue in wrenCall where return values could be prematurely collected by ensuring proper root handling.
- Refactor the call API test to avoid re-entering the VM by moving test execution into an afterLoad callback instead of a foreign method.

test/api/call.wren

@@ -27,11 +27,8 @@ class Api {
     // Otherwise print it.
     System.print(value)
   }
-
-  foreign static runTests()
 }
 
-Api.runTests()
 // expect: noParams
 // expect: zero
 // expect: one 1