Add full numerical convergence tests for all explicit tableaus

Revive the comprehensive convergence test from the original
DiffEqDevTools.jl (758dee9) that numerically verifies convergence
order for every explicit tableau from order 2 through 14 using
test_convergence with ExplicitRK solver.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas