[IO][Tests] Add basic test for the MeshSTLLoader

[Camille-K]

This PR adds a base test for the MeshSTLLoader component, based on similar tests for the OBJ and VTK loaders. It loads and checks the composition of the 3 .stl meshes available in share/meshes

Fix #2899

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[hugtalbot]

wizz @Camille-K

[alxbilger]

[ci-build][with-all-tests]