TestMemory, nee TestModify, is trying to test the PtraceByteBuffer. It doesn't need to. Instead a very simple portable memory I/O test, that confirms that the Proc's memory is correctly wired is required. A lower level sys test should be checking the PtraceByteBuffer.
One possible example might be to look in the AUXV for a known address and then read/modify that. The main thing though is that this really should be ISA independant and simple.
Other tests, such as code inserting/removing breakpoints are testing memory I/O.