test: improve tests structure with test cases in test_position_sequence

[tlambert03]

I'm getting closer to filing a finished PR for #232 and #192 ... but need to be able to confirm the original behavior a little better, so will be doing a few PRs to cleanup our testing framework

[codecov]

Codecov Report

Attention: Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.20%. Comparing base (7f7baa4) to head (f9a365c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/useq/experimental/protocols.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
- Coverage   97.54%   97.20%   -0.34%     
==========================================
  Files          22       22              
  Lines        1546     1576      +30     
==========================================
+ Hits         1508     1532      +24     
- Misses         38       44       +6     

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