Revamp the test_subscription.cpp tests. (#2227)

The original motiviation to do this was a crash during
teardown when using a newer version of gtest.  But while
I was in here, I did a small overall cleanup, including:

1.  Moving code closer to where it is actually used.
2.  Getting rid of unused 'using' statements.
3.  Adding in missing includes.
4.  Properly tearing down and recreating the rclcpp
    context during test teardown (this fixed the actual
    bug).
5.  Making class members private where possible.
6.  Renaming class methods to our usual conventions.

Signed-off-by: Chris Lalancette <[email protected]>