Add a process_vm_readv debug test

LLDB uses process_vm_readv to speed up memory reads (it's an order
of magnitude faster than PTRACE_PEEKDATA). Add a test to catch if
that ever breaks.

I've refactored the previous test a bit to reuse the
fork-and-run-stuff-in-the-child functionality.

Bug: b.android.com/227758
Test: cts-tradefed run cts --module CtsDebugTestCases
Change-Id: I7445b2997f8ef64b5540082ba0cb329827cf66de
2 files changed