Add back a modernized version of SolenoidTest and use it as unit test
After discussion with @jchapman decided to "re-introduce" the SolenoidTest
see for 24.0 version
[https://gitlab.cern.ch/atlas/athena/-/blob/24.0/MagneticField/MagFieldUtils/src/SolenoidTest.cxx]
And use it also for unit-testing.
What is added here is a slightly modernized version of the 24.0
code.
This one is done on top of !74257 (merged). So should be reviewed after that one is in
Edited by Christos Anastopoulos