[Heartbeat] Use `pytest_asyncio.fixture()`

[quachtridat]

Summary

Let conftest.py in cogs/heartbeat use pytest_asyncio.fixture() to define async fixtures, instead of pytest.fixture(). This helps resolve Injabie3/lui-cogs-v3#10 because pytest_asyncio.fixture() is necessary for async fixtures to be picked up by pytest-asyncio strict mode.

Testing

All tests ran and passed with pytest-asyncio strict test discovery mode.

PS> pytest .\cogs -o python_files=test*
================================================================== test session starts ================================================================== 
platform win32 -- Python 3.9.13, pytest-7.2.0, pluggy-1.0.0
rootdir: E:\Data\Coding\GitHub\quachtridat\Ren, configfile: pyproject.toml
plugins: aiohttp-json-rpc-0.13.3, asyncio-0.20.3, mock-3.10.0, Red-DiscordBot-3.5.1.dev1
asyncio: mode=strict
collected 75 items                                                                                                                                        

cogs\birthday\testBirthday.py .....                                                                                                                [  6%] 
cogs\birthday\testConverters.py ...........                                                                                                        [ 21%] 
cogs\heartbeat\testCommandsCore.py .......                                                                                                         [ 30%] 
cogs\heartbeat\testCore.py .                                                                                                                       [ 32%] 
cogs\tags\testHelpers.py ......................................                                                                                    [ 82%] 
cogs\wordfilter\testWordFilter.py .............                                                                                                    [100%]