Fix test breakage with text-2.1.2

[maurges]

Building with latest text you get the error:

> Building test suite 'integration' for yesod-auth-hashdb-1.7.1.7...
       > [1 of 4] Compiling TestSite         ( test/TestSite.hs, dist/build/integration/integration-tmp/TestSite.o )
       > test/TestSite.hs:134:18: error: [GHC-87543]
       >     • Ambiguous occurrence ‘show’.
       >       It could refer to
       >          either ‘Prelude.show’,
       >                 imported from ‘Prelude’ at test/TestSite.hs:18:8-15
       >                 (and originally defined in ‘ghc-internal-9.1003.0:GHC.Internal.Show’),
       >              or ‘Data.Text.show’,
       >                 imported from ‘Data.Text’ at test/TestSite.hs:28:1-16.
       >     • In the quasi-quotation:
       >         [whamlet|
       >             <p>Your current auth ID: #{show mname}
       >             $maybe _ <- mname
       >                 <p>
       >                     <a href=@{AuthR LogoutR}>Logout
       >             $nothing
       >                 <p>
       >                     <a href=@{AuthR LoginR}>Go to the login page
       >             <p><a href=@{ProtectedR}>Go to protected page
       >         |]
       >     |
       > 134 |         [whamlet|
       >     |                  ^...

Adding an explicit import list to Data.Text fixes the ambiguity

[Borgvall]

Nixpkgs has marked the package as broken due to this build error.