Fix a race condition in rest_client_SUITE

[arcusfelis]

This PR addresses

====== Suite FAILED: rest_client_SUITE (1 of 30 tests failed)    
                                                                 
                                                                 
                                                                 
====== Test name: add_contact_and_invite                         
====== Reason:    {{badmatch,                                    
                       #{ask => <<"out">>,                       
                         jid => <<"alice12.325484@localhost">>,  
                         subscription => <<"none">>}},           
                   [{rest_client_SUITE,'-add_contact_and_invite/1-fun-0-',2,                                                      
                        [{file,"rest_client_SUITE.erl"},{line,697}]},                                                             
                    {escalus_story,story,4,                      
                        [{file,"src/escalus_story.erl"},{line,72}]},                                                              
                    {rest_client_SUITE,add_contact_and_invite,1, 
                        [{file,"rest_client_SUITE.erl"},{line,643}]},                                                             
                    {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}]}]}

Proposed changes include:

• wait for processing confirmation.

[codecov]

Codecov Report

Merging #1813 into master will increase coverage by 7.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1813      +/-   ##
==========================================
+ Coverage   67.36%   74.68%   +7.32%     
==========================================
  Files         297      297              
  Lines       27746    27759      +13     
==========================================
+ Hits        18692    20733    +2041     
+ Misses       9054     7026    -2028

Impacted Files	Coverage Δ
src/mongoose_fips.erl	41.17% <0%> (-33.83%)	⬇️
...c/global_distrib/mod_global_distrib_server_mgr.erl	79.57% <0%> (ø)	⬆️
src/mod_pubsub.erl	66.01% <0%> (+0.06%)	⬆️
src/ejabberd_c2s.erl	84.35% <0%> (+0.14%)	⬆️
src/mod_mam.erl	89.43% <0%> (+0.37%)	⬆️
src/p1_fsm_old.erl	37.35% <0%> (+0.37%)	⬆️
src/mod_mam_odbc_arch.erl	89.34% <0%> (+0.4%)	⬆️
src/ejabberd_auth_odbc.erl	50.9% <0%> (+0.9%)	⬆️
src/mod_bosh_socket.erl	81.66% <0%> (+1%)	⬆️
src/mod_muc_light_utils.erl	85.26% <0%> (+1.05%)	⬆️
... and 44 more

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