Simplify test implementation and remove complex reusable code for better readability (replacement for #3510)

[heplesser]

This PR replaces #3510 because that PR had grown to 458 files due to some git mix-up. There is the original description:

This code was written by me [@Helveg] with a lot of initial enthusiasm for pytest during a hackathon experimenting with best patterns for pytest, and it completely missed its mark for providing an easy reusable simulation fixture. I've removed it in favor of straightforward, less complex, much more readable test code.

The tests included here map to SLI tests as follows. The tests were ported first at an earlier point in time and then revised @Helveg in June 2025 and are now re-revised. The original SLI tests covered by the PR were in NEST-3.2:

• For test_iaf_psc_alpha_fudge.py
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_fudge.sli
• For test_iaf_psc_dc_accurary.py
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_ps_dc_accuracy.sli
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_ps_dc_t_accuracy.sli
• For test_iaf_psc_alpha_1to2.py
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_1to2.sli
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_1to2_default_delay.sli
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_1to2_ex.sli
  • Notes
    • The "default_delay" variant is almost identical to the "plain" variant, it only uses the default delay instead of setting the delay explicitly to 1.0. That may have been useful to test in 2004 when the tests were first written, but this is checked elsewhere now, so that test is dropped.
    • The "ex" variant tests that setting "min_delay" to different values while keeping the actual delay constant does not affect results. This is now all subsumed in parameterization of a single test.
• For test_iaf_psc_alpha_mindelay.py
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_min_delay_create.sli
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_min_delay_set.sli
  • https://github.com/nest/nest-simulator/blob/v3.2/testsuite/unittests/test_iaf_min_delay_simblocks.sli

[terhorstd]

OK, this all looks okay to me now. Thanks for the nice discussion and the quick and thorough fixes.
🚀