[MLIR][Presburger] unittests: use an MLIRContext declared in parsePoly

Use an `MLIRContext` declared in a single place in the `parsePoly` function that almost all Presburger unit tests use for parsing sets. This function is only used in tests.

This saves us from having to declare and pass a new `MLIRContext` in every test.

Reviewed By: bondhugula, mehdi_amini

Differential Revision: https://reviews.llvm.org/D119251