[GHC] #11551: Get doctests into testsuite

[GHC]

#11551: Get doctests into testsuite
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        Reporter:  lwm               |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  7.10.3
      Resolution:                    |             Keywords:
                                     |  documentation, doctest
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by thomie):

 Go for it.

 Either create a separate `doctest` target in the testsuite (see
 `testsuite/Makefile`, `testsuite/mk/test.mk` and
 `testsuite/tests/Makefile`), and have `validate` call that. Or add it
 simply as a `run_command` test.

 Think about what should happen when the user doesn't have `doctest`
 installed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11551#comment:1>
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