testing/c: start a ADIOS2_C_API test fixture
testing/c: start a ADIOS2_C_API test fixture

It takes care of creating a adios2_adios* adiosH and MPI rank and size,
so that tests don't have to do this standard setup themselves.

JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2