test/librados: refactor setup/teardown in tiering tests

test/librados: refactor setup/teardown in tiering tests

These tests can either use the standard RadosTest, with one pool, or a
new test case with two pools.

The new test case has to copy a couple lines in static methods
(working around lack of virtual static methods is more complex than
it's worth here), but the rest is straight forward. Rename
base_pool_name and base_ioctx to pool_name and ioctx to match the
member variables of the parent class RadosTest.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>