Replace is_result with is_iq_result

[arcusfelis]

is_result is deprecated

This PR addresses

=INFO REPORT==== 8-Feb-2018::17:19:04 ===
calling deprecated function is_result, use is_iq_result instead
[{escalus_new_assert,assert,2,[{file,"src/escalus_new_assert.erl"},{line,31}]},
 {push_SUITE,'-muclight_no_msg_notifications_if_user_online/1-fun-1-',4,
             [{file,"push_SUITE.erl"},{line,477}]},
 {escalus_story,story,4,[{file,"src/escalus_story.erl"},{line,72}]},
 {test_server,ts_tc,3,[{file,"test_server.erl"},{line,1529}]},
 {test_server,run_test_case_eval1,6,[{file,"test_server.erl"},{line,1045}]},
 {test_server,run_test_case_eval,9,[{file,"test_server.erl"},{line,977}]}]

in CI tests