feat(test): add --dart-define

[Giuspepe]

Description

This PR adds the --dart-define parameter to the test command.

--dart-define is used to define compile-time constants which are then available through the String.fromEnvironment, bool.fromEnvironment, int.fromEnvironment, and double.fromEnvironment constructors.
Tests relying on such compile-time constants can currently not be run with very_good test, because the command doesn't yet accept the --dart-define parameter.

An example where one would use a --dart-define compile-time constant is to switch between different environments or flavors of an app. E.g. --dart-define=APP_ENVIRONMENT=dev.
This is useful because code as the following is automatically deleted in an optimization step when building an app:

const environment = String.fromEnvironment('APP_ENVIRONMENT');
const isEnvironmentDev = environment == 'dev';

...

// the whole if-block is automatically deleted during optimization when APP_FLAVOR != dev
// because `isEnvironmentDev` is a compile-time constant
if(isEnvironmentDev) {
  final secretDataOnlyForDev = 42;
}

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[Giuspepe]

FYI: It seems that the CI test task is a bit flaky. I got the following error on my first CI run. When I reran the CI task with the same code, it succeeded without any errors.
The errors don't look like they're caused by my changes though.

https://github.com/VeryGoodOpenSource/very_good_cli/runs/8308620438?check_suite_focus=true

[felangel]

LGTM thanks so much for the contribution!