Suppress all HealthChecks on test_split_datasets.

[bdice]

Follow-up to #5790. We suppressed HealthCheck.too_slow and HealthCheck.data_too_large, but another failure appeared with HealthCheck.filter_too_much. Instead, we should just skip all health checks for this test.

=========================== short test summary info ============================
FAILED test_strategies.py::test_split_datasets - hypothesis.errors.FailedHealthCheck: It looks like your strategy is filtering out a lot of data. Health check found 50 filtered examples but only 4 good ones. This will make your tests much slower, and also will probably distort the data generation quite a lot. You should adapt your strategy to filter less. This can also be caused by a low max_leaves parameter in recursive() calls
See https://hypothesis.readthedocs.io/en/latest/healthchecks.html for more information about this. If you want to disable just this health check, add HealthCheck.filter_too_much to the suppress_health_check settings for this test.
= 1 failed, 13454 passed, 5886 skipped, 632 xfailed, 54 xpassed, 10155 warnings in 1659.57s (0:27:39) =

[dantegd]

/merge