pytest warnings: add TESTFLAGS for pytest-asyncio to auto
Attempt to resolve globally during install the asyncio_mode warning
ult value will change to 'strict' in future, please explicitly use 'asyncio_mode=strict' or 'asyncio_mode=auto' in pytes
t configuration file.
config.issue_config_time_warning(LEGACY_MODE, stacklevel=2)```
Note: Doesn't take care of current builds. `echo "asyncio_mode=auto" >> swh-environment/<repo>/pytest.ini`
## Test Plan
I have set it to `auto` rather than `strict` to prevent any mishaps
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*Migrated from D7639 ([view on Phabricator](https://forge.softwareheritage.org/D7639))*