Some tests were still failing because different modules are writing in the same directories, using the same files names.
There are at least 3 different locations the tests write to:
1. build/ExternalData/Testing/Data/SystemTest
2. build/Testing/SystemTests/scripts
3. build
In some cases a process tries to write to a file that already exists (same name used by another test).
In other cases a process performs a .cleanup() which removes a file that is still needed by a running test.
The solution was to group the tests in lists where all modules starting with the same 4 letters are given to one
core. This worsens the load balance between cores (with 8 cores, core 1 performs 93 tests while core 8 only has 44).
This is not ideal but allows the suite to complete without failures.
The runtime using 8 cores goes down from 2h to 30min, which is ok as a first step.