chore: forgot to add gotest to _tools go.mod

[markphelps]

before this change, running mage bootstrap would return:

Bootstrapping tools...
no required module provides package github.com/rakyll/gotest; to add it:
        go get github.com/rakyll/gotest
Error: exit status 1

[codecov]

Codecov Report

Merging #1823 (fc0b697) into main (f4a8afd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1823   +/-   ##
=======================================
  Coverage   71.63%   71.63%           
=======================================
  Files          58       58           
  Lines        5500     5500           
=======================================
  Hits         3940     3940           
  Misses       1335     1335           
  Partials      225      225           

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