pkgs/build-support/cabal: filter internal 'testDepends' field from the expression to avoid unnecessary hash changes
pkgs/build-support/cabal: filter 'doCheck=false' field from the expression to avoid unnecessary hash changes

In the master branch, doCheck defaults to 'false', which means that no package
will change its hash unless its doCheck field is set to 'true' explicitly. In
the stdenv-updates branch, however, all Haskell packages have a default setting
of 'doCheck=true'. Once that branch has been merged, filtering doCheck is no
longer necessary.