test/librados: create general test case classes

test/librados: create general test case classes

Using a test case allows us to remove a boatload of boilerplate code
in all the tests, and focus them more on what they're actually
testing.

Create one for C tests, and one for C++ tests, with the same
functionality:

- create a pool when the test case starts
- between individual tests, create an ioctx and set its namespace uniquely
- delete objects from the default namespace during individual test teardown
- delete the pool only when the whole test case is finished

In gtest, a test case is the whole set of tests declared as part of
the same class using TEST_F(). Many tests create and delete individual
pools, but this is unnecessary for independent operation in most
cases, since we can use namespaces for that now. Since pool creation
and deletion dominates test run time, using these test cases makes
running many of the tests much faster.

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