Do not request tests for engine deletion-only PRs

[jmagman]

Matches framework repo:

cocoon/app_dart/lib/src/request_handlers/github/webhook_subscription.dart

Lines 241 to 249 in 68e36df

	await for (PullRequestFile file in files) {
	// When null, do not assume 0 lines have been added.
	final String filename = file.filename!;
	final int linesAdded = file.additionsCount ?? 1;
	final int linesDeleted = file.deletionsCount ?? 0;
	final int linesTotal = file.changesCount ?? linesDeleted + linesAdded;
	final bool addedCode = linesAdded > 0 || linesDeleted != linesTotal;
	if (addedCode &&

and packages repos:

cocoon/app_dart/lib/src/request_handlers/github/webhook_subscription.dart

Lines 463 to 468 in 68e36df

	final int linesAdded = file.additionsCount ?? 1;
	final int linesDeleted = file.deletionsCount ?? 0;
	final int linesTotal = file.changesCount ?? linesDeleted + linesAdded;
	final bool addedCode = linesAdded > 0 || linesDeleted != linesTotal;
	if (addedCode &&

Would have avoided requesting tests for flutter/engine#39682

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[jmagman]

Actually we don't need 3 copies of this logic, I'll DRY it out.