test: add an OSDMap unittest

test: add an OSDMap unittest

This is not super-sophisticated, but it does basic mapping function
consistency checks and looks at the [pg|primary]_temp manipulations. If
we want to in the future, we can do these programmatically across a range
of pgids instead of just checking hash 0.

Signed-off-by: Greg Farnum <greg@inktank.com>