Fix sporadic test failures for test_issue_36

[Nothing4You]

see e.g. https://github.com/aio-libs/aiomysql/runs/4995767021?check_suite_focus=true

restarting the CI job fixes the test run, something is not consistent there.

[Nothing4You]

additional context, as CI logs are volatile:

=================================== FAILURES ===================================
_____________________ test_issue_36[asyncio-tcp-mysql8.0] ______________________

connection_creator = <function connection_creator.<locals>.f at 0x7eff31f0cee0>

    @pytest.mark.run_loop
    async def test_issue_36(connection_creator):
        conn = await connection_creator()
        c = await conn.cursor()
        # kill connections[0]
        await c.execute("show processlist")
        kill_id = None
        rows = await c.fetchall()
        for row in rows:
            id = row[0]
            info = row[7]
            if info == "show processlist":
                kill_id = id
                break
        try:
            # now nuke the connection
            await conn.kill(kill_id)
            # make sure this connection has broken
            await c.execute("show tables")
            pytest.fail()
        except Exception:
            pass
    
        # check the process list from the other connection
        conn2 = await connection_creator()
        c = await conn2.cursor()
        await c.execute("show processlist")
        rows = await c.fetchall()
        ids = [row[0] for row in rows]
    
>       assert kill_id not in ids
E       assert 1018 not in [5, 1018, 1019]

tests/test_issues.py:258: AssertionError