Use pathlib for test path construction.

[delucchi-cmu]

Change Description

This PR touches only files under tests/, and changes code to use pathlib.Path for constructing test data paths, instead of os.path.join.

This shouldn't change any functionality, but is just cleaner.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.68%. Comparing base (6d427ac) to head (9597c79).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #289 +/- ## ======================================= Coverage 93.68% 93.68% ======================================= Files 57 57 Lines 1979 1979 ======================================= Hits 1854 1854 Misses 125 125 ```

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[github-actions[bot]]

Before [6d427ac5]	After [53844ed4]	Ratio	Benchmark (Parameter)
40.2±1ms	41.6±1ms	1.04	benchmarks.Suite.time_pixel_tree_creation
122±2ms	124±0.6ms	1.02	benchmarks.time_test_alignment_even_sky
12.9±0.2ms	13.1±0.2ms	1.01	benchmarks.Suite.time_inner_pixel_alignment
981±6μs	986±4μs	1.01	benchmarks.time_test_cone_filter_multiple_order
77.4±0.4ms	76.9±0.6ms	0.99	benchmarks.MetadataSuite.time_load_partition_info_order7
363±2ms	361±2ms	0.99	benchmarks.Suite.time_outer_pixel_alignment
77.6±0.9ms	76.3±0.2ms	0.98	benchmarks.MetadataSuite.time_load_partition_join_info
83.4±2ms	82.1±2ms	0.98	benchmarks.Suite.time_paths_creation
19.2±0.6ms	18.7±0.5ms	0.97	benchmarks.MetadataSuite.time_load_partition_info_order6

Click here to view all benchmarks.