do not rely on undefined behaviour in bytes_test.go

[ondrejbudai]

8 exbi equals 2^64, therefore it cannot be stored in int64. The tests use the fact that on x86_64 the following expressions holds true: int64(0) - 1 == math.MaxInt64.

However, this is not true for other platforms, specifically aarch64, s390x and ppc64le.

This commit fixes it by testing the library with 7 exbi.

Fixes #37

[codecov-commenter]

Codecov Report

Merging #38 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   59.26%   59.26%           
=======================================
  Files           6        6           
  Lines         518      518           
=======================================
  Hits          307      307           
  Misses        208      208           
  Partials        3        3           

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