Support custom item adapters in fixtures

[codecov[bot]]

Codecov Report

Merging #171 (36efbc9) into master (f4b58bd) will increase coverage by 0.01%. The diff coverage is 100.00%.

:exclamation: Current head 36efbc9 differs from pull request most recent head 74946a2. Consider uploading reports for the commit 74946a2 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #171 +/- ## ========================================== + Coverage 98.58% 98.59% +0.01% ========================================== Files 29 30 +1 Lines 1340 1356 +16 ========================================== + Hits 1321 1337 +16 Misses 19 19 ``` | [Impacted Files](https://codecov.io/gh/scrapinghub/web-poet/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub) | Coverage Δ | | |---|---|---| | [web\_poet/serialization/utils.py](https://codecov.io/gh/scrapinghub/web-poet/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvc2VyaWFsaXphdGlvbi91dGlscy5weQ==) | `100.00% <100.00%> (ø)` | | | [web\_poet/testing/fixture.py](https://codecov.io/gh/scrapinghub/web-poet/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvdGVzdGluZy9maXh0dXJlLnB5) | `99.43% <100.00%> (+0.03%)` | :arrow_up: | | [web\_poet/testing/itemadapter.py](https://codecov.io/gh/scrapinghub/web-poet/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapinghub#diff-d2ViX3BvZXQvdGVzdGluZy9pdGVtYWRhcHRlci5weQ==) | `100.00% <100.00%> (ø)` | |

[kmike]

Could you please check that https://web-poet.readthedocs.io/en/stable/page-objects/testing.html is still correct after introducing this feature?