test: use integers.json file to benchmark integer decoder

[tdakkota]

name                         old time/op    new time/op    delta
Decoder_Int/integers.json-4    3.61µs ± 2%    3.26µs ± 2%  -9.82%  (p=0.000 n=14+15)

name                         old alloc/op   new alloc/op   delta
Decoder_Int/integers.json-4     0.00B          0.00B         ~     (all equal)

name                         old allocs/op  new allocs/op  delta
Decoder_Int/integers.json-4      0.00           0.00         ~     (all equal)

[codecov]

Codecov Report

Merging #42 (1535c41) into main (8bfec7d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #42   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         2307      2306    -1     
=========================================
- Hits          2307      2306    -1     

Impacted Files	Coverage Δ
dec_int.go	100.00% <ø> (ø)

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