It would be really nice it the test task supported running with a custom system classloader. A typical use case would be the need to run tests with a WeavingURLClassLoader (aspectJ).

example:
dependencies

test

results in
"Error occurred during initialization of VM
java.lang.Error: java.lang.ClassNotFoundException: com.mycompany.system.test.MyWeavingURLClassLoader"

Comment from Adam;
"The way the test process is implemented means this won't work, as the project's classes aren't available on the system classpath when the jvm tries to instantiate the system classloader. There's no particular reason that this has to be the case, and we could certainly shuffle things around so that the classes are available."

Below a more detailed example of a custom test task to illustrate what would be nice to achieve with the gradle supplied test task;

build.gradle:
task testJunitJava(type: JavaExec) {
testReportsDir = new File(buildDir, "reports/tests")
doFirst

classpath = configurations.antJunitTest //classpath with required ant artifacts
main = "org.apache.tools.ant.Main"
args = ["-buildfile", new File(projectDir, "build-tests-ltw.xml")]
systemProperties("test.classpath":configurations.testCompile.asPath,
"target.test.aspects":new File(buildDir, "classes/test-aspects"),
"target.test.reports":testReportsDir
)
}

build-tests-ltw.xml (ant build file)
<project name="external-unittest" default="test" basedir=".">
<target name="test">
<!-- Defaults. Can be overridden on the command line -->
<property name="maven.test.include" value="**/*TestCase.class"/>
<property name="maven.test.exclude" value=""/>
<property name="weaving.class.loader" value="com.telenor.cosmos.system.test.CosmosWeavingURLClassLoader"/>

<junit printsummary="true" haltonfailure="no" fork="yes" forkmode="perBatch">
<jvmarg value="-Djavax.xml.transform.TransformerFactory=com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl"/>
<jvmarg value="-Djavax.xml.parsers.SAXParserFactory=com.sun.org.apache.xerces.internal.jaxp.SAXParserFactoryImpl"/>
<jvmarg value="-Djava.system.class.loader=${weaving.class.loader}"/>
<jvmarg value="-Djavax.xml.parsers.DocumentBuilderFactory=com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl"/>
<jvmarg value="-Xmx768m"/>
<jvmarg value="-XX:MaxPermSize=192m"/>
<classpath>
<pathelement path="${java.class.path}"/>
<pathelement path="${test.classpath}"/>
<pathelement path="c:\dev\tools\apache-ant-1.8.1\libs17\ant-junit.jar"/>
<pathelement path="target/classes/test-aspects"/>
<pathelement path="target/classes/test"/>
<pathelement path="target/classes/main"/>
</classpath>

<formatter type="xml"/>

<batchtest todir="${target.test.reports}" failureproperty="junit.failure">
<fileset dir="target/classes/test" includes="${maven.test.include}" excludes="${maven.test.exclude}"/>
</batchtest>
</junit>

<antcall target="writeJunitIfSet"/>
<antcall target="writeJunitIfNotSet"/>
</target>

<target name="writeJunitIfSet" if="junit.failure">
<echo file="target/junit.tmp" append="false">junit.failure=${junit.failure}
</echo>
</target>

<target name="writeJunitIfNotSet" unless="junit.failure">
<echo file="target/junit.tmp" append="false">#All tests passed
</echo>
</target>

</project>