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 ⚠️

Additional details and impacted files

@@           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.