Beef up process_vm_readv/writev tests

LLDB uses process_vm_readv quite extensively (it's an order of
magnitude faster than PTRACE_PEEKDATA). Add a bit more tests of
the function to make sure it stays working.

Change-Id: I64e17e6d56842f118a9485e3a18f42ca5e1b7577
Test: run the new test