Adapting WalberlaAccuracyTest for pipeline PreStage only

On closer inspection it is difficult to test the suspicion that
we run the _sp jobs of the pipeline with double, too. Problem
is that when this is the case the setting of the macro
WALBERLA_DOUBLE_ACCURACY will be wrong and we cannot use it
to distinguish between single and double precision as the
desired value.

Current Prestage has two jobs:
- clang_13.0_mpionly_eigen_petsc_trilinos
- gcc_11_mpionly_dbg_sp
so we can hardcode that for the moment to base decision on
the compiler identification.
31 jobs for mohr/pipeline-sp in 44 minutes (queued for 2 seconds)
Name Stage Failure
gcc_11_mpionly_dbg_sp Pretest
hyteg                    = 1217.13 sec*proc (212 tests)

Total Test time (real) = 1212.75 sec

The following tests FAILED:
213 - WalberlaAccuracyTest (Failed)
Errors while running CTest
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1