[MLIR] rewrite AffineStructures and Presburger tests to use the parser

This commit rewrites most existing unittests involving FlatAffineConstraints
to use the parsing utility. This helps to make the tests more understandable.

This relands commit b0e8667b, which was
reverted in 6963be12, with a fix to a unittest
which was incorrectly rewritten before.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D115920