LU-6714 llog: test on-disk llog header values

LU-6714 llog: test on-disk llog header values

llog_test_2():
- re-enable the disabled llog_open() test cases.
- Checks that llog_log_hdr values are written atomically
  with the llog record addition/cancelling.

Patch contains also minor fixes:
- verify_handle() does header sanity checks at first then
  checks amount of records against expected value.
- llog_test_3: rename test_3 static variables to show
  that they are related to the test_3.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: Iedcf15c8f365f9c2021abae3325edcaf08efc4c9
Reviewed-on: http://review.whamcloud.com/16287
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>