Fix solver_test.cc
Fix solver_test.cc

When Eigen is not installed, Solver::IsValid was not detecting
it correctly.

Change-Id: Id285a84d829a9e20bc5de663adfca66ac31e08f3

ysonggit/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,entylop/ceres-solver,stonier/ceres_solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,stonier/ceres_solver,stonier/ceres_solver,whuaegeanse/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,stonier/ceres_solver,kartikkumar/ceres-solver,kartikkumar/ceres-solver,whuaegeanse/ceres-solver,kartikkumar/ceres-solver,kartikkumar/ceres-solver,DougFirErickson/ceres-solver