[mlir][vector] Update tests for xfer-flatten (nfc)

Updates formatting and variable names in:
  * vector-transfer-flatten.mlir

This is primarily to improve consistency, both within this particular
test file as well as across tests.

Overview of changes:
  * All memref input arguments are re-named as `%mem`.
  * All vector input arguments are re-named as `%vec`.
  * All tensor input arguments are re-named as `%dest`.
  * LIT variables are update to be consistent with input arguments.
  * Renamed all output arguments as `%res`.
  * Updated indentation to be more C-like.

Note, similar changes for vector-transfer-permutation-lowering.mlir were
implemented in #101468.

Author: banach-space