Option to exclude returns by context

[imsodin]

When using contexts extensively, coverage tends to be flaky because different exit paths may be taken. Some sentry type selects look like this:

	select {
	case <-ctx.Done():
		return ctx.Err()
	default:
	}

however in the general case it's just on case in a bigger select statement:

	select {
	[...]
	case <-ctx.Done():
		return ctx.Err()
	}

Being able to filter that out would be really nice.