Check flaky test_benchmark_gradient / Weber_BMC2015

[dweindl]

Since roughly two weeks, there seem to occur more frequent failures during the finite differences check for the Weber_BMC2015 model (example).
Check, and fix seed or tolerances if necessary.

tests/benchmark_models/test_petab_benchmark.py::test_benchmark_gradient[Weber_BMC2015-forward-scaled] FAILED [ 81%]
tests/benchmark_models/test_petab_benchmark.py::test_benchmark_gradient[Weber_BMC2015-forward-unscaled] PASSED [ 82%]
tests/benchmark_models/test_petab_benchmark.py::test_benchmark_gradient[Weber_BMC2015-adjoint-scaled] FAILED [ 82%]
tests/benchmark_models/test_petab_benchmark.py::test_benchmark_gradient[Weber_BMC2015-adjoint-unscaled] PASSED [ 83%]