"graceful fetch" test should....clean up gracefully!

"graceful fetch" test should....clean up gracefully!

this test was relying on gc to close out the connection.
this would lead to problems with aiosqlite as we invalidate
async connetions that aren't gracefully closed, and this test
suite was create tables per suite, so we'd get into a spot where
a new sqlite memory connection without the tables would get set up.

would only occur for full test run + -n2 + C extensions + python 3.7,
but we assume that is all related to just getting gc to trigger
or not trigger at exactly the right moment in this situation.

confirmed if we add a gc.collect() to the test without explcitly
closing out the conenction, the connection is gc'ed and detached,
and we get the error reproduced on the subsequent test.

Change-Id: Icc9d4bc703f0842c27600f532f34bc4c7d3baf21