[nexus] Make it 'more default' for Debug datasets to exist in test env

[smklein]

Before this PR:

• DiskTest::new did not create any debug datasets
• omicron-dev run-all did not initialize any disks
• Any tests wanting debug datasets would need to explicitly request them, then call disk_test.propagate_datasets_to_sleds().
• Additionally, in this environment, many tests assume "the only tests which exist are crucible datasets".

After this PR:

• DiskTest::new does creates debug datasets on all zpools
• omicron-dev run-all initializes disks on the "first sled agent", with datasets
• Any tests using DiskTest::new will get them by default
• Tests are adjusted to cope with multiple dataset types

[smklein]

As an example of where this is useful:

#7972

We can create (and store) support bundles on the simulated system.

[smklein]

This isn't strictly necessary, but IMO it's a little clearer -- we're calling "add_zpool_with_dataset" to make a fourth zpool, on top of the three which already exist.