chore: Add test coverage analysis

[RolandBurrows]

Short description of what this PR does:

As a contributor, I'd like to know what baseline code coverage is and how my enhancements affect it.

No documentation was updated because code coverage is now part of the default rake test run, with no further action required.

smtpapi-ruby$ rake test

<test debug info>
Coverage report generated for Unit Tests, Unknown Test Framework,
test:units to ../smtpapi-ruby/coverage. 101 / 101 LOC (100.0%) covered.

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• (N/A) I have added tests that prove my fix is effective or that my feature works
• (N/A) I have added necessary documentation about the functionality in the appropriate .md file
• (N/A) I have added in line documentation to the code I modified

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @RolandBurrows,

Thanks again for the PR!

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