Tests fail to compile when enableSwagger is false

[j6carey]

To reproduce:

$ nix-build -A proto3-suite --arg enableSwagger false
...
tests/TestCodeGen.hs:18:1: error:
    Could not find module ‘Data.Swagger’
    Perhaps you meant
      Data.Tagged (needs flag -package-key tagged-0.8.6.1)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
18 | import           Data.Swagger                   (ToSchema)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/TestCodeGen.hs:19:1: error:
    Could not find module ‘Data.Swagger’
    Perhaps you meant
      Data.Tagged (needs flag -package-key tagged-0.8.6.1)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
19 | import qualified Data.Swagger
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^