Add tests for sensitive input

What does this MR do?

• Adds a test for the step-runner ci command. While the functionality is tested elsewhere, it serves and a good place to experiment because the test produces a step-results.json file (not checked in).
• Adds a test, non-sensitive input can derives value from sensitive output, to runner_test.go. This test passes, and will be changed in future to fail once the functionality is implemented (a non-sensitive input should not be able to derive its value using a sensitive output).
• Changes testCases in test_common.go so the t.Run happens in the same file as the test, this makes sure GoLand uses can run each test case directly from their IDE.

Reference

Sensitive outputs can only be given to sensitiv... (#94)