refactor(gtest): Change block execution model in gtest to be similar to the vara-runtime

[techraed]

Release notes:

1. Sending message to program simply means adding message to the queue, not executing block immediately as it was done previously. So program.send(...) now doesn't trigger block execution.
2. The block execution model is now similar to vara-runtime block execution. Block execution consists of 2 steps: scheduled tasks processing and message processing.
3. To start block execution one of the following methods must be called: run_next_block, run_next_block_with_allowance, run_to_block. Scheduled tasks can be processed separately without processing messages by calling run_scheduled_tasks.
4. Instead of RunResult a new BlockRunResult is introduced, which gives some report data of the block execution.
5. Sending signals from the Program API is removed as it violates the rules of the runtime: signals are sent only on panic automatically after failed execution, as it's done on the vara runtime (see bcba7c0).

TODO:

• resolve todos (open corresponding issues)
• fix workspace tests using gtest

[ByteNacked]

LGTM 👍