Test using __model__, world in @attached_to, @relative_to

[scpeters]

🦟 Bug fix

Counterpart to gazebosim/sdf_tutorials#77

Summary

As noted in gazebosim/sdf_tutorials#77, it was unclear in our documentation that __model__ and world can be used explicitly in the //frame/@attached_to and //pose/@relative_to attributes in the proper context. As that pull request updates the documentation, this pull request uses these values explicitly in test/sdf files and in corresponding integration tests.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1066 (1f336f6) into sdf9 (e9e9546) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 1f336f6 differs from pull request most recent head 96aec38. Consider uploading reports for the commit 96aec38 to get more accurate results

@@           Coverage Diff           @@
##             sdf9    #1066   +/-   ##
=======================================
  Coverage   87.59%   87.59%           
=======================================
  Files          64       64           
  Lines       10061    10061           
=======================================
  Hits         8813     8813           
  Misses       1248     1248           

Impacted Files	Coverage Δ
src/Model.cc	83.70% <ø> (ø)
src/parser.cc	79.05% <100.00%> (ø)

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