[mlir][linalg] Simplify padding test (NFC).

The padding tests previously contained the tile loops. This revision removes the tile loops since padding itself does not consider the loops. Instead the induction variables are passed in as function arguments which promotes them to symbols in the affine expressions. Note that the pad-and-hoist.mlir test still exercises padding in the context of the full loop nest.

Depends On D114175

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D114227