remove pub from to_packet_batches_for_tests

[apfitzge]

Problem

This shouldn't be pub because it is marked as cfg(test) and is generic.
If you try to use this outside of the crate, it leads to confusing messaging about the fn not existing.

Summary of Changes

Remove pub

Fixes #

[apfitzge]

A relevant rust-lang feature request PR: rust-lang/cargo#8379

[codecov]

Codecov Report

Merging #31056 (8f48bcc) into master (399b7ba) will increase coverage by 0.0%.
The diff coverage is 96.5%.

@@           Coverage Diff            @@
##           master   #31056    +/-   ##
========================================
  Coverage    81.5%    81.5%            
========================================
  Files         728      728            
  Lines      205473   205547    +74     
========================================
+ Hits       167525   167630   +105     
+ Misses      37948    37917    -31     

[steviez]

LGTM - can always re-promote to pub(crate) later if we end up wanting it somewhere else in this crate