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ARROW-7532: [CI] Unskip brew test after Homebrew fixes it upstream

Closes #6196 from nealrichardson/brew-test and squashes the following commits:

9b5361dfe <Neal Richardson> Don't fail on brew doctor
c32d20b30 <Neal Richardson> Add in brew config && brew doctor, in case of future issues
d1bfd66d5 <Neal Richardson> Reinstate brew test after upstream fix

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>