Fix no space error

[piobab]

1. Move tests to produce singe crate in test coverage (similar to Enable test coverage #305).

Moving to src caused circular dependency issue (see #344).

Before this PR:

➜ du -sh target/*         
4.0K    target/CACHEDIR.TAG
8.8G    target/coverage
144M    target/release
1.1G    target/wasm32-unknown-unknown

After this PR:

### after cargo coverage
➜ du -sh target/*         
4.0K    target/CACHEDIR.TAG
4.8G    target/coverage
144M    target/release
1.1G    target/wasm32-unknown-unknown

Reduction from 8.8 GB to 4.8 GB.

2. Change paths in coverage setup to reuse space from debug deps.

3. In coverage job after cargo make build remove all files except wasms.