Tests live in the tests/tests.bats file if you're adding a bugfix or making a small change, please ensure all tests pass.
To run the tests, call make from the project root. You'll need to have bats-core installed and in your $PATH.
If you only want to run a test or two, uncomment the # skip lines on the tests you don't care about, so they read skip, and then those tests will be skipped.
If you're thinking about new functionality, check that it can't be achieved by passing -o options to curl; for instance, there's no need to raise a PR that makes ttfb ignore invalid certificates, you can achieve this by calling ttfb like ttfb -o "-k" https://example.com - the -k option is added to the curl call, and will therefore ignore invalid or expired certificates.
If you do want to add new functionality, please also add a test to tests/tests.bats file.