test: test for config file creation and default config file parsing

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code passes tests (cargo test)

---

Closes #451

What does this PR do

1. Add test for:
   1. check the default configuration in config.example.toml is valid in config.rs (unit test)
   2. topgrade will create a default configuration file in ~/.config.topgrade.toml (CI)

Some questions

1. It seems that .github/workflow/test.yaml is not being run in the CI, only check-and-lint.yaml is.
2. Unit tests are not covered in the CI for the reason stated in 1

[s34m]

.github/workflow/test.yaml isn't activated right now because the only test in it was the code coverage test, which is useless rn, till now