[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:  bgamari
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Continuous        |              Version:  8.0.1
  Integration                        |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14890            |  Differential Rev(s):  Phab:D4354
       Wiki Page:                    |
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Changes (by bgamari):

 * related:   => #14890


Comment:

 We also need to fix the (numerous) known failures. See #14890.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13205#comment:4>
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