Add first Codecov test

[wilzbach]

Resends the CodeCov hooks received at druntime. I'm merging this immediately (if passing) as I want to try it out directly.

See also: #60

[dlang-bot]

Thanks for your pull request, @wilzbach!

[wilzbach]

Current status: As I'm currently trying to get the CodeCov settings, it seems getting CodeCov to succeed to matter what, can be done with a combination of:

• require_ci_to_pass: false
• after_n_builds: 1
• allow_coverage_offsets: true
• project: default: threshold: 0.1
• patch: default: informational: true

So we might not need to pursue forwarding coverage status reports from CodeCov.