yesod-persistent: workarount tests build failure

Test failed to build as:

> test/Yesod/PersistSpec.hs:16:1:
>     Illegal instance declaration for ‘ToBackendKey SqlBackend Person’
>       (All instance types must be of the form (T t1 ... tn)
>        where T is not a synonym.
>        Use TypeSynonymInstances if you want to disable this.)
>     In the instance declaration for ‘ToBackendKey SqlBackend Person’

Might not be the best fix, please check if it makes sense.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>