ZTS: Add reservation_008_pos exception

The reservation_008_pos test case has been observed to fail in
a non-dangerous way in approximately 5% of automated test runs.
Add the test case to the list of possible expected failures
until the test case can be made perfectly reliable.

Reviewed by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7741 
Closes #7742

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

@@ -250,6 +250,7 @@
     'pyzfs/pyzfs_unittest': ['SKIP', python_deps_reason],
     'no_space/enospc_002_pos': ['FAIL', enospc_reason],
     'projectquota/setup': ['SKIP', exec_reason],
+    'reservation/reservation_008_pos': ['FAIL', '7741'],
     'reservation/reservation_018_pos': ['FAIL', '5642'],
     'rsend/rsend_019_pos': ['FAIL', '6086'],
     'rsend/rsend_020_pos': ['FAIL', '6446'],