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

tests/test-runner/bin/zts-report.py

@@ -105,6 +105,14 @@ user_ns_reason = 'Kernel user namespace support required'
 #
 rewind_reason = 'Arbitrary pool rewind is not guaranteed'
 
+#
+# Some tests may by structured in a way that relies on exact knowledge
+# of how much free space in available in a pool.  These tests cannot be
+# made completely reliable because the internal details of how free space
+# is managed are not exposed to user space.
+#
+enospc_reason = 'Exact free space reporting is not guaranteed'
+
 #
 # Some tests are not applicable to Linux or need to be updated to operate
 # in the manor required by Linux.  Any tests which are skipped for this
@@ -235,8 +243,7 @@ maybe = {
     'inuse/inuse_009_pos': ['SKIP', disk_reason],
     'largest_pool/largest_pool_001_pos': ['FAIL', known_reason],
     'pyzfs/pyzfs_unittest': ['SKIP', python_deps_reason],
-    'no_space/setup': ['SKIP', disk_reason],
-    'no_space/enospc_002_pos': ['FAIL', known_reason],
+    'no_space/enospc_002_pos': ['FAIL', enospc_reason],
     'projectquota/setup': ['SKIP', exec_reason],
     'reservation/reservation_018_pos': ['FAIL', '5642'],
     'rsend/rsend_019_pos': ['FAIL', '6086'],