XSLT: Split and merge XML elements

I'm a beginner in XSLT programming. I've got the task to transform the following xml:

<Test>TestA::test1</Test>
<Test>TestA::test2</Test>
<Test>TestB::test3</Test>
<Test>TestB::test4</Test>

The output xml shall look like this:

<TestClass id="TestA">
    <TestMethod id="test1"/>
    <TestMethod id="test2"/>
</TestClass>
<TestClass id="TestB">
    <TestMethod id="test3"/>
    <TestMethod id="test4"/>
</TestClass>

I've tried a lot of different approaches and read myriad stackoverflow threds, but couldn't find a solution.

Thanks in advance, mexl