Add testing primitives for BDD-style tests

[rdw-software]

Goals:

• [x] The testing primitives are exported to the global environment (and unit tested)
• [x] The custom assertions are exported to the global environment (and unit tested)
• [x] Running a scenario triggers the documented event handlers
• [x] Running a test suite results in the expected console output
• [x] Running a scenario results in the expected console output
  • [x] No assertions
  • [x] Standard assertion without a description
  • [x] Standard assertion with a description
  • [x] Nonstandard assertions without a description
  • [x] Nonstandard assertions with a description
  • [x] Multiple assertions, some succeeding and others failing
• [x] The documented usage example is especially tested
• [x] Failing tests exit with a nonzero code to enable testing as part of the CI pipeline

[rdw-software]

Failing tests exit with a nonzero code to enable testing as part of the CI pipeline

This is best handled by wrapping the TestSuite:Run() method with a standard assert. It will return a status flag indicating whether any scenario has failed. I guess that's good enough and doesn't require further hacking stuff in that isn't otherwise useful...

[rdw-software]

The documented usage example is especially tested

I'm checking this off too, since it's guaranteed to run without errors... but more detailed testing needs to be performed for all code snippets that are published in the documentation (docs issue).