test: ensure 95+% test coverage

[sergey-melnychuk]

Current test coverage is ~50% (excluding binary and generated code), fulfilling test gaps and ensuring we're as close to 100% as we can has utmost importance now. (Though 95% should be acceptable for now).

#815 (comment)
#815

https://github.com/eigerco/beerus/blob/main/doc/coverage.md