chore: lance-encoding test utility now properly aligns buffers

[westonpace]

We align buffers in the file writer but we were not doing the same thing in the test utility. This forced encodings to do extra copies. We remove one such copy in this PR.

Closes #3115

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.13%. Comparing base (c237bcb) to head (b5117b9).

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/encoder.rs	16.66%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3116 +/- ## ========================================== + Coverage 77.11% 77.13% +0.01% ========================================== Files 240 240 Lines 80643 80652 +9 Branches 80643 80652 +9 ========================================== + Hits 62190 62213 +23 + Misses 15275 15274 -1 + Partials 3178 3165 -13 ``` | [Flag](https://app.codecov.io/gh/lancedb/lance/pull/3116/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/lancedb/lance/pull/3116/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | `77.13% <66.66%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb#carryforward-flags-in-the-pull-request-comment) to find out more.

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[broccoliSpicy]

some benchmark results related to this PR: before eliminating copy:

after eliminating copy: