tests: Make tests runnable from anywhere

[l0b0]

By using the current file rather than the root of the project as the
reference, the tests are now runnable for example in an IDE which uses
the "tests" directory as the root.

Related Issue(s): #331

Description:

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #496 (a7c0e43) into main (f9573f8) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
- Coverage   91.66%   91.64%   -0.02%     
==========================================
  Files          40       40              
  Lines        5245     5245              
==========================================
- Hits         4808     4807       -1     
- Misses        437      438       +1     

Impacted Files	Coverage Δ
pystac/stac_object.py	94.17% <0.00%> (-0.53%)	⬇️

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