Rename original numerics/derivative_approximation tests for consistency

diff --git a/tests/numerics/derivative_approximation.cc b/tests/numerics/derivative_approximation_01.cc
similarity index 100%
rename from tests/numerics/derivative_approximation.cc
rename to tests/numerics/derivative_approximation_01.cc

diff --git a/tests/numerics/derivative_approximation.output b/tests/numerics/derivative_approximation_01.output
similarity index 100%
rename from tests/numerics/derivative_approximation.output
rename to tests/numerics/derivative_approximation_01.output

diff --git a/tests/numerics/derivative_approximation_2.cc b/tests/numerics/derivative_approximation_02.cc
similarity index 100%
rename from tests/numerics/derivative_approximation_2.cc
rename to tests/numerics/derivative_approximation_02.cc

diff --git a/tests/numerics/derivative_approximation_2.output b/tests/numerics/derivative_approximation_02.output
similarity index 100%
rename from tests/numerics/derivative_approximation_2.output
rename to tests/numerics/derivative_approximation_02.output