[MLIR][NFC] Move test/Transforms/lower-affine.mlir -> test/Conversion

Move lower-affine.mlir from test/Transforms to
test/Conversion/AffineToStandard/. Other related NFC.

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D77008