tests: JSON comparison command for LabN topotests

tests: JSON comparison command for LabN topotests

The changes add the "jsoncmp_pass" and the "jsoncmp_fail" commands to
compare VTY shell's JSON output to an expected JSON object during
topotests using the LabN testing framework. This helps to eliminate
false negative test results (i.e. due to routes beeing out of order
after convergence or cosmetic changes in VTY shell's text output).

Signed-off-by: David Schweizer <dschweizer@opensourcerouting.org>