TestSolution is dense even if the solution itself is not

[ChrisRackauckas-Claude]

Migrated from SciML/DiffEqDevTools.jl#96 (reported by @nathanaelbosch).

TestSolution always reports dense=true even when constructed from a non-dense solution:

using OrdinaryDiffEq, DiffEqDevTools
import ODEProblemLibrary: prob_ode_linear
prob = prob_ode_linear

solve(prob, Tsit5(), dense=false).dense          # false ✓
TestSolution(solve(prob, Tsit5(), dense=false)).dense  # true ✗

The TestSolution constructor unconditionally sets dense=true based on the presence of t and u fields, rather than checking the source solution's dense flag.

Verified still present as of 2026-03-30.