Code coverage in CI

[aslamplr]

Add unit test modules

[aslamplr]

Overall 66 % coverage
> 98% coverage for gh-lib crate.
0% for gh-auth.
0% for gh-cli.

[aslamplr]

Screenshots of HTML output from tarpaulin run locally
docker run --rm --security-opt seccomp=unconfined -v ${PWD}:/volume xd009642/tarpaulin cargo tarpaulin --out Html

/

/gh-lib/src/

/gh-lib/src/core

/gh-lib/src/utils

Note: So far only happy path test cases have been added