[Enhancement] Add proper feature testing library/framework

[cschot]

The CLI provides a useful, but limited testing library for developers, called dev-container-features-test-lib to use in test.sh

I propose to add a proper, widely used bash library or framework, so testing can be more reliable.

Suggestions about which framework or library to use, or which possibilities it should have are welcome.

I myself think of 2 options:

1. A library like the one that is used now, but much more reliable and extended. The code is run first, and than the result is checked by assertion commands.
2. A Unit testing framework that can be used for to test the code in isolated parts. It does not run the code first, but checks every function separately.

I think option 1 is fine for now. But if there is demand for Unit testing, we better integrate it right away.