It turns out the test results are only correctly reported if the test name is scoped with the class name. So the original XML fails:
<?xml version="1.0" encoding='ISO-8859-1' standalone='yes' ?>
<TestRun>
<FailedTests>
</FailedTests>
<SuccessfulTests>
<Test id="1">
<Name>AlwaysPassTest</Name>
</Test>
</SuccessfulTests>
<Statistics>
<Tests>1</Tests>
<FailuresTotal>0</FailuresTotal>
<Errors>0</Errors>
<Failures>0</Failures>
</Statistics>
</TestRun>
And this slightly updated version passes:
<?xml version="1.0" encoding='ISO-8859-1' standalone='yes' ?>
<TestRun>
<FailedTests>
</FailedTests>
<SuccessfulTests>
<Test id="1">
<Name>SampleTest::AlwaysPassTest</Name>
</Test>
</SuccessfulTests>
<Statistics>
<Tests>1</Tests>
<FailuresTotal>0</FailuresTotal>
<Errors>0</Errors>
<Failures>0</Failures>
</Statistics>
</TestRun>