Testing framework: add support for context saving/restoring

type:added

This MR adds support for saving/restoring the context in testing framework:

Test.save_context : unit -> unit
Test.restore_context : unit -> unit

This could be useful for doing testing of contracts, you set up a context, and then try different transactions on the very same context instead of resetting the whole context.

Changelog details:

New operations Test.save_context and Test.restore_context

• Test.save_context : unit -> unit : takes current testing framework context and saves it, pushing it into a stack of contexts.
• Test.restore_context : unit -> unit : pops a testing framework context from the stack of contexts, and sets it up as the new current context. In case the stack was empty, the current context is kept.