Create strong unit tests for all collection types.

[Dandielo]

The tests in questions need to cover all functions, and complexity possibilities.

• string, heap_string, static_string
• array, queue, hashmap
• shard_container

Additionally nice to have are extended tests for:

• span and hashmapview

_Originally posted by @Dandielo in https://github.com/iceshard-engine/engine/pull/139#discussion_r1001747269_

[Dandielo]

Make really robust tests for string find_* operations.