Add testing helper module

[smcguire-cmu]

Closes #646

[github-actions]

Before [93e8451]	After [cbe972f]	Ratio	Benchmark (Parameter)
1.11±0.02ms	1.27±0.1ms	~1.14	benchmarks.time_test_cone_filter_multiple_order
12.7±0.5ms	12.8±0.5ms	1.01	benchmarks.Suite.time_inner_pixel_alignment
5.64±0.4ms	5.65±0.5ms	1.00	benchmarks.ConeFilterSuite.time_cone_filter_benchmark
119±0.9ms	120±0.6ms	1.00	benchmarks.time_test_alignment_even_sky
41.6±1ms	41.2±0.5ms	0.99	benchmarks.Suite.time_pixel_tree_creation
504±3ms	499±2ms	0.99	benchmarks.time_open_large_catalog
77.7±0.2ms	76.9±0.2ms	0.99	benchmarks.time_open_midsize_catalog
506±2ms	499±2ms	0.99	benchmarks.time_small_cone_large_catalog
414±2ms	406±2ms	0.98	benchmarks.Suite.time_outer_pixel_alignment

Click here to view all benchmarks.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (93e8451) to head (e56b474).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hats/testing/helpers.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
- Coverage   92.54%   92.24%   -0.31%     
==========================================
  Files          51       53       +2     
  Lines        2684     2696      +12     
==========================================
+ Hits         2484     2487       +3     
- Misses        200      209       +9     

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