Adjust test assertions to fix flakiness (gradle#8979)

This fixes gradle/gradle-private#1760

The changes we made to TAPI in gradle#7654 caused some flakiness.
This PR fixes these flakiness by adjusting test assertions.

subprojects/tooling-api/src/crossVersionTest/groovy/org/gradle/integtests/tooling/CancellationSpec.groovy

@@ -22,6 +22,7 @@ import org.gradle.integtests.tooling.fixture.TestResultHandler
 import org.gradle.integtests.tooling.fixture.ToolingApiSpecification
 import org.gradle.test.fixtures.server.http.BlockingHttpServer
 import org.gradle.tooling.BuildCancelledException
+import org.gradle.util.GradleVersion
 import org.junit.Rule
 
 import java.util.concurrent.CountDownLatch
@@ -72,12 +73,20 @@ latch.await()
     void buildWasCancelled(TestResultHandler resultHandler, String failureMessage = 'Could not execute build using Gradle') {
         resultHandler.assertFailedWith(BuildCancelledException)
         assert resultHandler.failure.message.startsWith(failureMessage)
-        assert resultHandler.failure.cause.message == "Build cancelled."
+
+        if (targetIsGradle51OrLater()) {
+            // https://github.com/gradle/gradle-private/issues/1760
+            assert resultHandler.failure.cause.message in ["Build cancelled.", "Daemon was stopped to handle build cancel request."]
+        }
         def failure = OutputScrapingExecutionFailure.from(stdout.toString(), stderr.toString())
         failure.assertHasDescription('Build cancelled.')
         assertHasBuildFailedLogging()
     }
 
+    private static void targetIsGradle51OrLater() {
+        targetVersion >= GradleVersion.version('5.1')
+    }
+
     void configureWasCancelled(TestResultHandler resultHandler, String failureMessage) {
         resultHandler.assertFailedWith(BuildCancelledException)
         assert resultHandler.failure.message.startsWith(failureMessage)