Handle no retry and only successful tests case

Fixes #38

Author: ldaley

plugin/src/main/java/org/gradle/testretry/internal/RetryTestExecuter.java

@@ -82,7 +82,7 @@ public void execute(JvmTestExecutionSpec spec, TestResultProcessor testResultPro
     }
 
     void failWithNonRetriedTestsIfAny() {
-        if (extension.getSimulateNotRetryableTest() || !lastResult.nonRetriedTests.isEmpty()) {
+        if (extension.getSimulateNotRetryableTest() || (lastResult != null && !lastResult.nonRetriedTests.isEmpty())) {
             throw new IllegalStateException("org.gradle.test-retry was unable to retry the following test methods, which is unexpected. Please file a bug report at https://github.com/gradle/test-retry-gradle-plugin/issues" +
                 lastResult.nonRetriedTests.stream()
                     .map(retry -> "   " + retry.getClassName() + "#" + retry.getName())

plugin/src/test/groovy/org/gradle/testretry/CorePluginFuncTest.groovy

@@ -39,6 +39,33 @@ class CorePluginFuncTest extends AbstractGeneralPluginFuncTest {
         gradleVersion << GRADLE_VERSIONS_UNDER_TEST
     }
 
+    @Unroll
+    def "is benign when unconfigured (gradle version #gradleVersion)"() {
+        when:
+        successfulTest()
+        gradleRunner(gradleVersion).build()
+
+        then:
+        assertTestReportContains("SuccessfulTests", reportedTestName("successTest"), 1, 0)
+
+        where:
+        gradleVersion << GRADLE_VERSIONS_UNDER_TEST
+    }
+
+    @Unroll
+    def "does not retry by default (gradle version #gradleVersion)"() {
+        when:
+        failedTest()
+        def result = gradleRunner(gradleVersion).buildAndFail()
+
+        then:
+        result.output.contains("There were failing tests.")
+        assertTestReportContains("FailedTests", reportedTestName("failedTest"), 0, 1)
+
+        where:
+        gradleVersion << GRADLE_VERSIONS_UNDER_TEST
+    }
+
     @Unroll
     def "retries failed tests (gradle version #gradleVersion)"() {
         given: