handling empty or missing snapshots

[nbujic-jnpr]

Description of Issue/Question

Hi,

If it happens that snapshot file is non-existing/empty, following issues can happen:

1. for missing/empty post snapshot test will be always skipped, which is different compared to empty pre-snapshot where missing data/nodes can be identifed and test will fail in that case (if not ignore-null is set)
2. for missing/empty pre snapshot if one of 4 "compare" checks is used, since in code data from post-snapshot is assigned to pre-snapshot test will pass reporting that there are no differences

Steps to Reproduce Issue 1.

• execute -snap pre
• execute --snap post and remove some of the snapshot files or use command which will generate empty output
• execute --compare pre post

# jsnapy --snap pre
Connecting to device 10.220.1.222 ................
Taking snapshot of COMMAND: show chassis hardware
# jsnapy --check pre post
**************************** Device: 10.220.1.222 ****************************
Tests Included: test_chassis_hardware
*********************** Command: show chassis hardware ***********************
ERROR, Snapshot file /root/jsnapy/snapshots/10.220.1.222_22_post_show_chassis_hardware.xml is not present in given path !!
------------------------------- Final Result!! -------------------------------
test_chassis_hardware : Skipped
Total No of tests passed: 0
Total No of tests failed: 0
None of the test cases executed !!!

Steps to Reproduce Issue 2.

• execute only --snap post or execute --snap pre and remove some of the related snapshot files
• execute --check pre post with tests containing "compare" operators

# jsnapy --snap post
Connecting to device 10.220.1.222 ................
Taking snapshot of COMMAND: show chassis hardware
# jsnapy --check pre post
**************************** Device: 10.220.1.222 ****************************
Tests Included: test_chassis_hardware
*********************** Command: show chassis hardware ***********************
ERROR, Snapshot file /root/jsnapy/snapshots/10.220.1.222_22_pre_show_chassis_hardware.xml is not present in given path !!
PASS | All "serial-number" is same in pre and post snapshot [ 1 value matched ]
------------------------------- Final Result!! -------------------------------
test_chassis_hardware : Passed
Total No of tests passed: 1
Total No of tests failed: 0
Overall Tests passed!!!

[chidanandpujar]

Fixed via #414