Only configure whitebox test suite if the a test src folder exists

This is mainly for the default 'test' source set, which is always
registered. If it is not used, the whitebox configuration will cause
Gradle to think that there are supposed to be tests, because the
main classes become part of the "test classes" set. This will cause
Gradle to run the task without tests and throw an error
(in recent Gradle versions).

Author: jjohannes

src/main/java/org/gradlex/javamodule/testing/JavaModuleTestingExtension.java

@@ -63,12 +63,15 @@ public JavaModuleTestingExtension(Project project, JavaModuleDetector moduleDete
         testing.getSuites().withType(JvmTestSuite.class).configureEach(jvmTestSuite -> {
             boolean isTestModule = jvmTestSuite.getSources().getJava().getSrcDirs().stream().anyMatch(src -> new File(src, "module-info.java").exists());
             if ("test".equals(jvmTestSuite.getName())) {
-                jvmTestSuite.useJUnitJupiter(); // override old Gradle default to default to JUnit5 for all suites
+                jvmTestSuite.useJUnitJupiter(); // override old Gradle convention to default to JUnit5 for all suites
             }
             if (isTestModule) {
                 blackbox(jvmTestSuite);
             } else {
-                whitebox(jvmTestSuite, conf -> conf.getOpensTo().add("org.junit.platform.commons"));
+                boolean testFolderExists = jvmTestSuite.getSources().getJava().getSrcDirs().stream().anyMatch(File::exists);
+                if (testFolderExists) {
+                    whitebox(jvmTestSuite, conf -> conf.getOpensTo().add("org.junit.platform.commons"));
+                }
             }
         });
     }