[mlir] Move TestDialect and its passes to mlir::test namespace.

TestDialect has many operations and they all live in ::mlir namespace.
Sometimes it is not clear whether the ops used in the code for the test passes
belong to Standard or to Test dialects.

Also, with this change it is easier to understand what test passes registered
in mlir-opt are actually passes in mlir/test.

Differential Revision: https://reviews.llvm.org/D90794