Add AHT20 tests

[SoulKa]

• Add tests for all exported methods of the AHT20 device (except SenseContinuous())
• Fix IsInitialized() always returning false due to data byte not being set

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e644b7c) 61.6% compared to head (e5f433a) 61.4%.

Files	Patch %	Lines
aht20/aht20.go	57.1%	0 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #70 +/- ## ======================================= - Coverage 61.6% 61.4% -0.2% ======================================= Files 65 67 +2 Lines 7442 7567 +125 ======================================= + Hits 4583 4646 +63 - Misses 2705 2758 +53 - Partials 154 163 +9 ```

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[maruel]

Thanks! If you want one optional thing that is sweet is to add an example that can be copy pasted by using package aht20_test. See other example files for an example and the corresponding generated doc.