Add a new test for sAHP extraction

[AurelienJaquier]

Motivation: sAHP has two amplitudes, amp and amp2, which was not tested with current tests (only extracting for IDRest)

• add init in tests folder so that we can fetch functions from other files, and exclude tests folder in setup find_packages, so that pypi does not make user download tests as a module, now that it has an init file in it.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d4edc58) 65.95% compared to head (d54e131) 70.22%.

Files	Patch %	Lines
tests/test_extractor.py	97.67%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #163 +/- ## ========================================== + Coverage 65.95% 70.22% +4.26% ========================================== Files 34 35 +1 Lines 2159 2223 +64 ========================================== + Hits 1424 1561 +137 + Misses 735 662 -73 ```

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