Fixed external values initialization in unit tests.

Fixed external values initialization in unit tests.

Since 0.8.0 modules can create their own constructors and prototypes.
A modules has two method: preinit() and init(). A module should
add its constructors and prototypes in preinit() and create its own
values in init(). Creating a value in preinit() results in an error.
The patch fixes the issue by creating an external value in init()
instead of preinit().

Found by UndefinedBehaviorSanitizer.