[mlir][linalg][nfc] Fix formatting for linalg.elementwise

Follow-up to #124661 to address minor formatting inconsistencies.

**Changes:**
1. Standardized test file names by using hyphens (`-`) instead of
   underscores (`_`).
2. Renamed `"round-trip.mlir"` to `"roundtrip.mlir"` for consistency
   with similar tests.
3. Normalized indentation.

For **(3)**, I ensured we follow the pre-existing formatting style
introduced with `linalg.generic`, where all new lines are indented by
**8 spaces**.  Thus, the formatting is now:

```mlir
  %r = linalg.elementwise
      kind=#linalg.elementwise_kind<sub>
      indexing_maps = [affine_map<(d0, d1) -> (d1, d0)>,
                       affine_map<(d0, d1) -> (d0, d1)>,
                       affine_map<(d0, d1) -> (d0, d1)>]
      ins(%A, %B: tensor<8x16xf32>, tensor<16x8xf32>)
      outs(%C: tensor<16x8xf32>) -> tensor<16x8xf32>
```

instead of:

```mlir
  %r = linalg.elementwise
               kind=#linalg.elementwise_kind<sub>
               indexing_maps = [affine_map<(d0, d1) -> (d1, d0)>,
                                affine_map<(d0, d1) -> (d0, d1)>,
                                affine_map<(d0, d1) -> (d0, d1)>]
               ins(%A, %B: tensor<8x16xf32>, tensor<16x8xf32>)
               outs(%C: tensor<16x8xf32>) -> tensor<16x8xf32>
```

Submitting this without a review, as these are straightforward changes,
and I want to reduce PR traffic/noise. However, please let me know if
you prefer changes like these to go through a formal PR review.

0 files changed, 0 insertions, 0 deletions