TestDiagnostic#getMessage behavior changed

[wmdietl]

#758 added DetailedTestDiagnostic for test diagnostics from -Adetailedmsgtext.
This unfortunately breaks clients that manually parsed this output.
TestDiagnostic#getMessage used to return the whole message, including the $$-separated arguments.
DetailedTestDiagnostic#getMessage only returns the readable part of the error message.

This could be fixed a few different ways, e.g. by changing the behavior of getMessage back to the original and adding a new DetailedTestDiagnostic#getReadableMessage, or by adding a new TestDiagnostic#getRawMessage or something like that.