Fixes #N/A.

Closes #3321

Summary/Motivation:

This is a small PR to test that we are actually testing all NEOS AMPL solvers. As part of this, it moves the logic for identifying all compatible NEOS solvers to a public method on the kestrelAAMPL interface.

By checking that we are testing against all advertised NEOS AMPL solvers, we can close #3321 (if Octeract is added back to the list of advertised solvers, then the test will start failing and we can revisit that issue).

Changes proposed in this PR:

• Add kestrelAMPL.getAvailableSolvers()
• Add a test that verifies RunAllNEOSSolvers has a test method for every solver.

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