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 ⚠️

Additional details and impacted files

@@            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	Coverage Δ
unittests	77.13% <66.66%> (+0.01%)	⬆️

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[broccoliSpicy]

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

after eliminating copy: