More consistency in the ZFS test suite.

Move all invocations of verify_disk_count into the top of the ATF test body.

tests/sys/cddl/zfs/include/libtest.kshlib
Fix the set_disks function when only one disk is available.
Previously, its API differed depending on whether 1 or >1 disks were
available. This didn't make sense, and all tests were developed
with >1 disks.

tests/sys/cddl/zfs/tests//*.ksh
tests/sys/cddl/zfs/tests//*_test.sh
Move many verify_disk_count invocations from individual test files
into the ATF script.

tests/sys/cddl/zfs/tests/cache/*.ksh
Fix all these tests. They were using verify_disk_count in a useless
way.

tests/sys/cddl/zfs/tests/cache/cache_010_neg.ksh
Make this test work more like the other cache tests.

Sponsored by: Spectra Logic Corp