print table content on error message

Examples:

not ok 58       TestNGCPRealPrefs:test_callee_usr_load_multi
    ./tests/ngcp_rp.lua:183: expected: {3,2,1}, actual: 3

not ok 82       TestNGCPXAvp:test_xavp_get_all
    ./tests/ngcp_xavp.lua:98: expected: {testid={2,1}}, actual: {testid=2}