[mlir][emitc] Update and extend the TOSA -> EmitC test (#177339)

This patch updates and extends the TOSA-to-EmitC lowering test:
  * Conversion/ConvertToEmitC/tosa.mlir

Summary of changes and rationale:
* Remove `buffer-alignment=0` from the lowering pipeline; it is not required
  (the existing `CHECK` lines are not affected).
* Move the test from Conversion/ConvertToEmitC/tosa.mlir to
  Dialect/EmitC/tosa/ops.mlir. Conversion tests are intended for single
  conversion passes (e.g. `-convert-dialect1-to-dialect2`), whereas this test
  exercises a more complex lowering pipeline with multiple explicit steps (e.g.
  TOSA -> Linalg, bufferization, etc.).
* Add a Transform Dialect sequence to complement the existing lowering pipeline
  definition. This introduces an additional `RUN` line that is compatible with
  the original one. Using the Transform Dialect makes the pipeline easier to
  document, maintain, and experiment with.