Add test config option to delete conflicting outputs by default in Dart 2

[smaifullerton-wk]

While Dart 1 is still the default modus operandi for devs, the Dart 2 build will prompt the user to delete conflicting outputs in repos with generated files. Currently, it's not possible to use a ddev task to run tests and delete these conflicting outputs.

This PR add a test config option to delete the conflicting outputs by default so we can turn on this option in highly trafficked repos and allow devs to use ddev test to run tests in Dart 2 without worrying about the underlying build stuff going on.

QA +1 Instructions

• CI passes
• Use dart_dev dep override in a repo that has generated files from a source_gen builder
• Set config.test.deleteConflictingOutputs = true in dev.dart of this repo
• Switch to Dart 2
• Confirm that you can run ddev test successfully, and that --delete-conflicting-outputs is added to the flags passed to the initial build

[aviary-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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Questions or Comments? Reach out on Slack: #support-infosec.

[todbachman-wf]

+10

Testing Performed

• CI (mostly) passes
• Use dart_dev dep override in a repo that has generated files from a source_gen builder
• Set config.test.deleteConflictingOutputs = true in dev.dart of this repo
• Switch to Dart 2
• Confirm that you can run ddev test successfully, and that --delete-conflicting-outputs is added to the flags passed to the initial build

[evanweible-wf]

@Workiva/release-management-p