ZTS: Test case failures

• large_dnode_008_pos - Force a pool sync before invoking zdb to ensure the updated dnode blocks have been persisted to disk.

• refreserv_raidz - Wait for the /dev/zvol links to be both created and removed, this is important because the same device volume names are being used repeatedly.

• btree_test - Add missing .gitignore file for btree_test binary.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9769