ZTS: Improve enospc tests

The enospc_002_pos test case would frequently fail due a command
succeeding when it was expected to fail due to lack of space.
In order to make this far less likely, files are created across
multiple transaction groups in order to consume as many unused
blocks as possible.

The dependency that the tests run on a partitioned block device
has been removed. It's simpler to use sparse files.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7663