test: buffer.indexOf: add more number cases

[nektro]

add some more cases that assert the success result is relative to the start of the Buffer and not the byteOffset parameter
existing number cases only checked 0 and -1 returns

discovered by oven-sh/bun#17555

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.31%. Comparing base (22ac597) to head (8edf69e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #57200   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files         630      630           
  Lines      184513   184513           
  Branches    36073    36071    -2     
=======================================
+ Hits       166644   166647    +3     
+ Misses      10962    10959    -3     
  Partials     6907     6907           

see 33 files with indirect coverage changes