Address testing related feedback

Expose inner VM externals as an unsafe method and move logic into the tests

lib/vm/src/lib.rs

@@ -44,6 +44,7 @@ pub use crate::global::*;
 pub use crate::imports::Imports;
 pub use crate::instance::{
     ImportFunctionEnv, ImportInitializerFuncPtr, InstanceAllocator, InstanceHandle,
+    WeakOrStrongInstanceRef,
 };
 pub use crate::memory::{LinearMemory, Memory, MemoryError, MemoryStyle};
 pub use crate::mmap::Mmap;