[mlir][vector] Refactor vector-transfer-unroll.mlir (NFC)

Overview of changes:

- All memref input arguments are re-named to %mem.
- All vector input arguments are re-named to %vec.
- LIT variables are update to be consistent with input arguments.
- Renamed all output arguments as %res.
- Unified identation of `FileCheck` commands
- Separated tests (previously there was a big test containing a couple of tests)

Author: pabloantoniom