From the doc:

There are four ways to mark a test as legacy:

• (Recommended) Add the @group legacy annotation to its class or method;
• Make its class name start with the Legacy prefix;
• Make its method name start with testLegacy instead of test;
• Make its data provider start with provideLegacy or getLegacy.

So for marking a test method as legacy, we add the Legacy part which results testLegacy. However PHPUnit test methods can also me snake case, i.e. test_legacy should also work (that is not the case).