[mlir][OpDSL] Simplify index and constant tests.

Simplify tests that use `linalg.fill_rng_2d` to focus on testing the `const` and `index` functions. Additionally, cleanup emit_misc.py to use simpler test functions and fix an error message in config.py.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D120734