Add a test for loading gltf with external textures

[luca-della-vedova]

🎉 New feature

Summary

This PR is a followup of #393 and as discussed in #393 (comment) it adds a test to make sure loading GLTF meshes with external textures works as well.

Test it

It's a new unit test so it will be handled by CI.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 #409 (92be243) into gz-common5 (aec75f5) will increase coverage by 0.02%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           gz-common5     #409      +/-   ##
==============================================
+ Coverage       80.54%   80.56%   +0.02%     
==============================================
  Files              85       85              
  Lines            9956     9956              
==============================================
+ Hits             8019     8021       +2     
+ Misses           1937     1935       -2     

Impacted Files	Coverage Δ
graphics/src/AssimpLoader.cc	89.01% <0.00%> (+0.75%)	⬆️

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[mjcarroll]

Use joinPaths rather than explicit slashes.

[mjcarroll]

A small nit, but if we are including an image, can it be the gazebo logo?

Addressed