gdb: re-work parallel-for-selftests.c

I find this file difficult to work with and modify, due to how it uses
the preprocessor to include itself, to generate variations of the test
functions.  Change it to something a bit more C++-y, with a test
function that accepts a callback to invoke the foreach function under
test.

Change-Id: Ibf1e2907380a88a4f8e4b4b88df2b0dfd0e9b6c8