Add more tests for "end-to-end" features

[joelgallant]

There is a gap in tests for checking that internal features are exposed to end users correctly. The unit tests of these features are decent, but any gaps when "putting it all together" aren't really tested.

In v1, I had a lot of these tests. I don't like them being all over the place, though. Outlining a list of these features, and putting these kinds of tests in one place is probably best.

Very related are CLI flags. Again, these are mostly covered, but by no means checked individually (like aliases, interactions of different flags, etc).

• Usage of parsing extensions
  • All default parsing extensions
  • Include extensions via meta file
• CLI options for vars and create subcommands (config loading options)
• CLI options for exec subcommand
• Scenarios with .app-config.secrets and .app-config files
• Scenarios with APP_CONFIG_EXTENDS and APP_CONFIG
• Scenarios with secrets in main files
• TODO