Add 'populated_dataset' fixture as alternative to 'populate_dataset' helper

[adswa]

fixes #64

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.20% :tada:

Comparison is base (8b76391) 96.93% compared to head (c804f5d) 97.14%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #67 +/- ## ========================================== + Coverage 96.93% 97.14% +0.20% ========================================== Files 15 15 Lines 196 210 +14 ========================================== + Hits 190 204 +14 Misses 6 6 ``` | [Files Changed](https://app.codecov.io/gh/datalad/datalad-ria/pull/67?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [datalad\_ria/conftest.py](https://app.codecov.io/gh/datalad/datalad-ria/pull/67?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9yaWEvY29uZnRlc3QucHk=) | `100.00% <ø> (ø)` | | | [datalad\_ria/tests/fixtures.py](https://app.codecov.io/gh/datalad/datalad-ria/pull/67?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9yaWEvdGVzdHMvZml4dHVyZXMucHk=) | `97.61% <100.00%> (+0.47%)` | :arrow_up: | | [datalad\_ria/tests/test\_fixtures.py](https://app.codecov.io/gh/datalad/datalad-ria/pull/67?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9yaWEvdGVzdHMvdGVzdF9maXh0dXJlcy5weQ==) | `100.00% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[adswa]

any objections to this PR?

[jsheunis]

No from my side