Avoid orphan flexible instances in Cabal-TestSuite documentation

[bos]

(Imported from Trac #814, reported by guest on 2011-03-13)

The example code shown in
http://www.haskell.org/cabal/release/cabal-1.10.1.0/doc/users-guide/#test-suites
needs FlexibleInstances?, and the instance is orphan, too.
Although this code is not intended to be used in most packages,
it still looks like those instances are necessary or at least good style.
I suggest to replace the generic pair by a custom data type like

data TestCase = TestCase String Bool
instance TestOptions TestCase where
   ...

in the documentation.

Since Cabal is a widely used library, it has automatically an educational aspect.

[bos]

(Imported comment by @ttuegel on 2011-03-13)

This bug will likely be fixed anyway when the new detailed test interface is released, but I'm assigning it to myself so it isn't forgotten.

[ttuegel]

This hasn't been part of the User Guide in a long time.