update docstring for assign_tasks_per_compute_unit and add check

[smoors]

fixes #330

[casparvl]

Looks good to me. Small test case shows nothing is broken:

$ reframe -c test-suite/eessi/testsuite/tests/apps/gromacs.py --run -t CI -t 1_8_node --system snellius:rome
/gpfs/home4/casparl/EESSI/eessi_test_venv/lib64/python3.9/site-packages/setuptools_scm/_integration/setuptools.py:31: RuntimeWarning:
ERROR: setuptools==53.0.0 is used in combination with setuptools_scm>=8.x

Your build configuration is incomplete and previously worked by accident!
setuptools_scm requires setuptools>=61

Suggested workaround if applicable:
 - migrating from the deprecated setup_requires mechanism to pep517/518
   and using a pyproject.toml to declare build dependencies
   which are reliably pre-installed before running the build tools

  warnings.warn(
[ReFrame Setup]
  version:           4.10.0-dev.0+04f8df5a
  command:           '/gpfs/home4/casparl/EESSI/eessi_test_venv/bin/reframe -c test-suite/eessi/testsuite/tests/apps/gromacs.py --run -t CI -t 1_8_node --system snellius:rome'
  launched by:       casparl@tcn317.local.snellius.surf.nl
  working directory: '/gpfs/home4/casparl/EESSI'
  settings files:    '<builtin>', '/gpfs/home4/casparl/EESSI/test-suite/config/surf_snellius.py'
  selected system:   'snellius'
  check search path: (R) '/gpfs/home4/casparl/EESSI/test-suite/eessi/testsuite/tests/apps/gromacs.py'
  stage directory:   '/scratch-shared/casparl/reframe_output/staging'
  output directory:  '/gpfs/home4/casparl/EESSI/reframe_runs/output'
  log files:         '/gpfs/home4/casparl/EESSI/reframe_runs/logs/reframe_20260506_131801.log'
  results database:  [off] '/home/casparl/.reframe/reports/results.db'

== Temporary log file in case of crash /scratch-local/casparl.22483237/eb-splau28r/easybuild-qw0fd5jp.log
[==========] Running 2 check(s)
[==========] Started on Wed May  6 13:18:04 2026+0200

[----------] start processing checks
[ RUN      ] EESSI_GROMACS %benchmark_info=HECBioSim/Crambin %nb_impl=cpu %scale=1_8_node %module_name=GROMACS/2025.2-foss-2025a /d39a9259 @snellius:rome+default
[ RUN      ] EESSI_GROMACS %benchmark_info=HECBioSim/Crambin %nb_impl=cpu %scale=1_8_node %module_name=GROMACS/2025.4-foss-2025b /be7d4945 @snellius:rome+default
[       OK ] (1/2) EESSI_GROMACS %benchmark_info=HECBioSim/Crambin %nb_impl=cpu %scale=1_8_node %module_name=GROMACS/2025.4-foss-2025b /be7d4945 @snellius:rome+default
P: perf: 77.097 ns/day
[       OK ] (2/2) EESSI_GROMACS %benchmark_info=HECBioSim/Crambin %nb_impl=cpu %scale=1_8_node %module_name=GROMACS/2025.2-foss-2025a /d39a9259 @snellius:rome+default
P: perf: 78.094 ns/day
[----------] all spawned checks have finished

[  PASSED  ] Ran 2/2 test case(s) from 2 check(s) (0 failure(s), 0 expected failure(s), 0 skipped, 0 aborted)
[==========] Finished on Wed May  6 13:20:45 2026+0200
Log file(s) saved in '/gpfs/home4/casparl/EESSI/reframe_runs/logs/reframe_20260506_131801.log'

Approved!