Add testminimal and a new testsuite case.

[johnfea]

Description

This PR adds a new testsuite case that seemed more convenient to add with a new test program instead of modifying testshade. However, if testshade can be easily modified to add this testsuite case then this PR might not be needed, however as is testshade doesn't even call "closure->as_comp()". Also, testrender doesn't support batched mode so that one can't be used for this testsuite case either without major modifications.

Additionally, OSL has no minimal sample and this PR adds one. I needed a minimal sample when I started using OSL.

Also, #1800 and #1801 need new testsuite cases that would be easy to add with testminimal but not necessarily so with testshade.

Tests

I added a new testsuite case "closure-string" that fails in AVX2 batched mode and also with non-batched icc/icx.

1831 should fix the failed test cases so this would need to be accepted after that one.

Checklist:

• [x] I have read the contribution guidelines.
• [ ] I have updated the documentation, if applicable.
• [x] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [x] My code follows the prevailing code style of this project. If I haven't already run clang-format v17 before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.