tests: correct mesages for skipping tests involving diffpy.srreal

[ycexiao]

What problems does this PR address?

Some tests are skipped because diffpy.srreal is unavailable, but the messages said it is because diffpy.structure is unavailable.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.60%. Comparing base (09a8e1c) to head (4531343).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_pdf.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage   66.60%   66.60%           
=======================================
  Files          25       25           
  Lines        3171     3171           
=======================================
  Hits         2112     2112           
  Misses       1059     1059           

Files with missing lines	Coverage Δ
tests/test_pdf.py	43.29% <0.00%> (ø)

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