[Test] Fix reproducibility of values used in add_species_sequential test

Exact floating point equality can be assured only in the case where the species
are added in the same order, since this affects summations involved in
calculating the mixture molecular weight. This resulted in test failures with
certain versions of the Intel compiler.

Resolves Cantera#433.

test/data/kineticsfromscratch.cti

@@ -2,7 +2,7 @@ units(length='m', time='s', quantity='kmol', act_energy='cal/mol')
 
 ideal_gas(name = "ohmech",
       elements = " O  H  Ar ",
-      species = """ h2o2: H2  H  O  O2  OH  H2O  HO2  H2O2  AR""",
+      species = """ h2o2: AR  O  H2  H  OH  O2  H2O  H2O2  HO2""",
       reactions = "all",
       transport = "None",
       initial_state = state(temperature = 300.0,