Skip test based on content of scico/data/examples

[bwohlberg]

Skip test based on content of scico/data/examples rather than content of of data directory.

Still to be confirmed that this modified skip test still achieves the original goal of skipping the tests for scico.data when run within a cloned repo without the scico-data submodule present.

Resolves #98.

[codecov]

Codecov Report

Merging #99 (a9330b5) into main (4684909) will not change coverage.
The diff coverage is n/a.

❗ Current head a9330b5 differs from pull request most recent head 27932aa. Consider uploading reports for the commit 27932aa to get more accurate results

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   90.17%   90.17%           
=======================================
  Files          42       42           
  Lines        2981     2981           
=======================================
  Hits         2688     2688           
  Misses        293      293           

Flag	Coverage Δ
unittests	90.17% <ø> (ø)

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[FernandoDavis]

Changed skipif condition to match general case. Works as expected in a new repo.