TestSolution is dense even if the solution itself is not

```
julia> solve(prob, Tsit5()).dense
true

julia> solve(prob, Tsit5(), dense=false).dense
false

julia> TestSolution(solve(prob, Tsit5())).dense
true

julia> TestSolution(solve(prob, Tsit5(), dense=false)).dense
true
```

This is unexpected to me, and might be a bug.