Better unit testing with workspaces

[totikom]

Our unit tests works as follows:

1. Copy 'tests/test-lib' to target
2. Build 'target/test-lib' (this invokes our macro and creates 'target/test-lib/fuzz' directory with different fuzz-targets)
3. Build each fuzz-target in 'target/test-lib/fuzz/fuzz_targets'

The disadvantage of this approach is that if macro produces code, which fails to compile, all the tests will fail.
Alternative is to create not just a crate 'test-lib', but a set of workspaces, which (1) can be compiled independently (2) shares a common dependencies cache.