feature: run inner tests with cargo

Tarantool specificities make it difficult to set up usual rust tests. Picolib is coupled with tarantool symbols and compiled as a dylib, so it can't be linked to a stand-alone binary.

This patch introduces another way of testing.

All tests (called inner) are already built into dylib and exposed in the tarantool console:

tarantool> picolib.test
---
- test_version: 'function: 0x41f32400'
  test_traft_storage_RaftEntryRow: 'function: 0x41f32550'
...

The new integration test inner.rs iterates over them and runs each one in a separate tarantool instance.

See also: commit 6610e5cb

Close #21 (closed)