Comparison of Einstein-Boltzmann solvers for testing general relativity by Bellini, E, Barreira, A, Frusciante, N, Hu, B, Peirone, S, Raveri, M, Zumalacárregui, M, Avilez-Lopez, A, Ballardini, M, Battye, R, Bolliet, B, Calabrese, E, Dirian, Y, Ferreira, P, Finelli, F, Huang, Z, Ivanov, M, Lesgourgues, J, Li, B, Lima, N, Pace, F, Paoletti, D, Sawicki, I, Silvestri, A, Skordis, C, Umiltà, C, Vernizzi, F