same issue here.

from the debug log:

24872 21:21:02.807 [DEBUG] [org.gradle.api.internal.tasks.testing.detection.ClassFileExtractionManager] extracted class groovy/lang/Closure from groovy-all-1.7.10.jar
24873 21:21:02.871 [DEBUG] [org.gradle.api.internal.tasks.testing.detection.ClassFileExtractionManager] extracted class groovy/lang/GroovyObjectSupport from groovy-all-1.7.10.jar
24874 21:21:02.880 [DEBUG] [org.gradle.api.internal.tasks.testing.detection.ClassFileExtractionManager] extracted class java/lang/Object from org.osgi.foundation-1.0.0.jar

I just ran into this. In my case, I don't have a direct dependency on org.osgi.foundation. I depend on org.osgi.compendium which pulls in foundation transitively.

I'm brand new to gradle, interested in moving off of maven for a multi-module OSGi project[1]. If we can't run the tests, my investigation is more or less stalled.

For a specific module where you can see this error, see [2] and use build.gradle at [3].

[1] https://github.com/sakaiproject/nakamura
[2] https://github.com/sakaiproject/nakamura/tree/master/bundles/connections
[3] http://pastebin.com/pfDwrbku

I also have this issue, if no @Test annoation in a java file under src/test/java and also the project has dependency to "org.osgi.foundation" jar, this issue will happen, now I temporarily solve it by exclude dependency to "org.osgi.foundation" like follow in my gradle.build file.

configurations {
  all*.exclude module: "org.osgi.foundation"
}

This almost four year old issue is still unresolved in Gradle 2.4.
IMHO it should have higher priority since it could prevent larger projects to switch to Gradle.

I got the same error today.
It happens because it's examining the class hierarchy and doesn't stop at java.lang.Object.
I think it can be solved by changing one line in org.gradle.api.internal.tasks.testing.detection.AbstractTestFrameworkDetector#getSuperTestClassFile, to exclude all JDK classes.
See the last return statement:

    protected File getSuperTestClassFile(String superClassName) {
        prepareClasspath();
        if (StringUtils.isEmpty(superClassName)) {
            throw new IllegalArgumentException("superClassName is empty!");
        }

        final Iterator<File> testClassDirectoriesIt = testClassDirectories.iterator();

        File superTestClassFile = null;
        while (superTestClassFile == null && testClassDirectoriesIt.hasNext()) {
            final File testClassDirectory = testClassDirectoriesIt.next();
            final File superTestClassFileCandidate = new File(testClassDirectory, superClassName + ".class");
            if (superTestClassFileCandidate.exists()) {
                superTestClassFile = superTestClassFileCandidate;
            }
        }

        if (superTestClassFile != null) {
            return superTestClassFile;
        } else { // super test class file not in test class directories
            // Skip JDK classes
            return superClassName.startsWith("java/") || superClassName.startsWith("javax/") ? null : classFileExtractionManager.getLibraryClassFile(superClassName);
        }
    }

For future reference, there was an unfinished pull request that contain some discussion at https://github.com/gradle/gradle/pull/558. It would be great if someone from the community could pick it up.

PR fixing this issue: https://github.com/gradle/gradle/pull/676