Use mock sampling throughout statespace tests (#518) #1857

Job	Run time
ubuntu (ubuntu-latest, 3.11, tests/statespace --ignore tests/statespace/core/test_statespace.py -...	7m 28s
ubuntu (ubuntu-latest, 3.11, tests/statespace/core/test_statespace.py)	6m 6s
ubuntu (ubuntu-latest, 3.11, tests/distributions)	6m 23s
ubuntu (ubuntu-latest, 3.11, tests/statespace/filters/test_kalman_filter.py)	2m 33s
windows (windows-latest, 3.12, tests --ignore tests/model --ignore tests/statespace --ignore test...	23m 44s
windows (windows-latest, 3.12, tests/statespace --ignore tests/statespace/core/test_statespace.py...	12m 5s
ubuntu (ubuntu-latest, 3.11, tests --ignore tests/model --ignore tests/statespace --ignore tests/...	14m 5s
ubuntu (ubuntu-latest, 3.11, tests/model)	9m 2s
windows (windows-latest, 3.12, tests/model)	14m 5s
windows (windows-latest, 3.12, tests/distributions)	10m 47s
windows (windows-latest, 3.12, tests/statespace/core/test_statespace.py)	10m 20s
windows (windows-latest, 3.12, tests/statespace/filters/test_kalman_filter.py)	5m 19s
all_tests	3s
	2h 2m 0s

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