[GHC] #13205: Run `validate --slow` during CI at least sometimes.

[GHC]

#13205: Run `validate --slow` during CI at least sometimes.
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           Reporter:  dobenour       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 We really should run `validate --slow` at times.  It would catch #13204.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13205>
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