Add Dart and Flutter CI systems to CI tests check.

The current check is looking at the github checks data to identify
whether a given PR ran tests. Flutter and Dart repos are failing the
check becuase their systems are not recognized as CI Systems.

Bug: ossf#1547

checks/ci_tests.go

@@ -151,6 +151,7 @@ func isTest(s string) bool {
 	for _, pattern := range []string{
 		"appveyor", "buildkite", "circleci", "e2e", "github-actions", "jenkins",
 		"mergeable", "packit-as-a-service", "semaphoreci", "test", "travis-ci",
+                "flutter-dashboard", "Cirrus CI",
 	} {
 		if strings.Contains(l, pattern) {
 			return true