Fix registrar test for Solaris.

[andrewkroh]

And generally change the test to provide more verbose errors on failure.
I was tired of seeing 🔴 for the Solaris Jenkins job.

Python stat does not return the same device number as command line stat and Go. So I changed the test case to not compare the st_dev value with the device value from the registry file.

Solaris stat:

vagrant@solaris$ stat log/test.log 
  File: ‘log/test.log’
  Size: 60              Blocks: 2          IO Block: 512    regular file
Device: 11700010007h/1198295941127d     Inode: 6519        Links: 1
Access: (0644/-rw-r--r--)  Uid: (  100/ vagrant)   Gid: (   10/   staff)
Access: 2016-03-29 17:59:04.449807589 +0000
Modify: 2016-03-29 17:59:04.320733252 +0000
Change: 2016-03-29 17:59:04.320733252 +0000
 Birth: -

Python (st_dev is the device number):

>>> print os.stat("test.log")
posix.stat_result(st_mode=33188, st_ino=6519L, st_dev=73203719L, st_nlink=1, st_uid=100, st_gid=10, st_size=60L, st_atime=1459274344, st_mtime=1459274344, st_ctime=1459274344)

Registrar file created by Filebeat (Go):

{
  "/export/home/vagrant/go/src/github.com/elastic/beats/filebeat/build/system-tests/run/test_registrar.Test.test_registrar_file_content/log/test.log": {
    "source": "/export/home/vagrant/go/src/github.com/elastic/beats/filebeat/build/system-tests/run/test_registrar.Test.test_registrar_file_content/log/test.log",
    "offset": 60,
    "FileStateOS": {
      "inode": 10589,
      "device": 1198295941127
    }
  }
}