Tests: hp/solution_transfer_15: add mpirun=1 to test variant

diff --git a/tests/hp/solution_transfer_15.with_trilinos=true.with_trilinos_with_zoltan=true.output b/tests/hp/solution_transfer_15.with_trilinos=true.with_trilinos_with_zoltan=true.mpirun=1.output
similarity index 100%
rename from tests/hp/solution_transfer_15.with_trilinos=true.with_trilinos_with_zoltan=true.output
rename to tests/hp/solution_transfer_15.with_trilinos=true.with_trilinos_with_zoltan=true.mpirun=1.output