TSAN: data race in `tests/test_ch.c`, global variable `session_established_flag`

[jktjkt]

Please use, e.g., an atomic variable for this.

WARNING: ThreadSanitizer: data race (pid=3845)
  Write of size 4 at 0x000001952de0 by thread T9:
    #0 ch_new_session_set_flag_cb /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/tests/test_ch.c:408:30 (test_ch+0x58b0ae) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #1 nc_server_ch_client_thread_session_cond_wait /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3337:9 (test_ch+0x51d0db) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #2 nc_ch_client_thread /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3574:17 (test_ch+0x516fe0) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)

  Previous read of size 4 at 0x000001952de0 by thread T8:
    #0 server_thread_delete_while_session /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/tests/test_ch.c:433:13 (test_ch+0x58aedd) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)

  As if synchronized via sleep:
    #0 usleep <null> (test_ch+0x45552b) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #1 nc_accept_ssh_session_open_netconf_channel /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server_ssh.c:1957:9 (test_ch+0x56b95f) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #2 nc_accept_ssh_session /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server_ssh.c:2201:15 (test_ch+0x56b104) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #3 nc_connect_ch_endpt /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3219:15 (test_ch+0x51cb98) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #4 nc_ch_client_thread /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3562:19 (test_ch+0x516ec8) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)

  Location is global 'session_established_flag' of size 4 at 0x000001952de0 (test_ch+0x1952de0)

  Thread T9 (tid=4052, running) created by thread T8 at:
    #0 pthread_create <null> (test_ch+0x45879f) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #1 _nc_connect_ch_client_dispatch /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3825:14 (test_ch+0x516b4b) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #2 nc_connect_ch_client_dispatch /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/src/session_server.c:3867:10 (test_ch+0x5181c1) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #3 server_thread_delete_while_session /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/tests/test_ch.c:429:11 (test_ch+0x58aeaf) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)

  Thread T8 (tid=4051, running) created by main thread at:
    #0 pthread_create <null> (test_ch+0x45879f) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #1 test_nc_ch_delete_client_while_session /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/tests/test_ch.c:553:11 (test_ch+0x589527) (BuildId: 5199417ab5ed6ab22fc3ee98039cfa2ce7f6d72d)
    #2 <null> <null> (libcmocka.so.0+0x618f) (BuildId: 785844a0941c0bde763740a981d056f60aa9c7b7)
    #3 __libc_start_call_main /usr/src/debug/glibc-2.37-18.fc38.x86_64/csu/../sysdeps/nptl/libc_start_call_main.h:58:16 (libc.so.6+0x27b89) (BuildId: 70e92bb237883be3065a6afc9f0696aef2d068bf)

SUMMARY: ThreadSanitizer: data race /home/ci/src/cesnet-gerrit-public/github/CESNET/libnetconf2/tests/test_ch.c:408:30 in ch_new_session_set_flag_cb

[michalvasko]

Right, fixed by @Roytak.