[mlir] Expand prefixing to OpFormatGen

Follow up to also use the prefixed emitters in OpFormatGen (moved
getGetterName(s) and getSetterName(s) to Operator as that is most
convenient usage wise even though it just depends on Dialect). Prefix
accessors in Test dialect and follow up on missed changes in
OpDefinitionsGen.

Differential Revision: https://reviews.llvm.org/D112118