tests: allocate ports to bind to via CMake

This fixes parallel test execution. Without this patch, parallel runs
were dying all the time for me (I'm using -j16 because that's what I
have even on this laptop).

This patch is the reason why I was converting the test suite to use
one CMake function call per test.
diff --git a/tests/test_ch.c b/tests/test_ch.c
index 396b310..6714e0f 100644
--- a/tests/test_ch.c
+++ b/tests/test_ch.c
@@ -152,7 +152,7 @@
     assert_int_equal(ret, 0);
 
     /* add call-home bind */
-    ret = nc_client_ssh_ch_add_bind_listen("127.0.0.1", 10009);
+    ret = nc_client_ssh_ch_add_bind_listen("127.0.0.1", TEST_PORT);
     assert_int_equal(ret, 0);
 
     pthread_barrier_wait(&state->barrier);
@@ -160,7 +160,7 @@
     ret = nc_accept_callhome(NC_ACCEPT_TIMEOUT, NULL, &session);
     assert_int_equal(ret, 1);
 
-    ret = nc_client_ssh_ch_del_bind("127.0.0.1", 10009);
+    ret = nc_client_ssh_ch_del_bind("127.0.0.1", TEST_PORT);
     assert_int_equal(ret, 0);
 
     nc_session_free(session, NULL);
@@ -198,7 +198,7 @@
     assert_int_equal(ret, 0);
 
     /* set call-home address and port */
-    ret = nc_server_config_add_ch_address_port(ctx, "ch_ssh", "endpt", NC_TI_LIBSSH, "127.0.0.1", "10009", &test_state->ssh_tree);
+    ret = nc_server_config_add_ch_address_port(ctx, "ch_ssh", "endpt", NC_TI_LIBSSH, "127.0.0.1", TEST_PORT_STR, &test_state->ssh_tree);
     assert_int_equal(ret, 0);
 
     /* set connection type to persistent */
@@ -333,7 +333,7 @@
     assert_int_equal(ret, 0);
 
     /* add call-home bind */
-    ret = nc_client_tls_ch_add_bind_listen("127.0.0.1", 10010);
+    ret = nc_client_tls_ch_add_bind_listen("127.0.0.1", TEST_PORT_2);
     assert_int_equal(ret, 0);
 
     pthread_barrier_wait(&state->barrier);
@@ -341,7 +341,7 @@
     ret = nc_accept_callhome(NC_ACCEPT_TIMEOUT, NULL, &session);
     assert_int_equal(ret, 1);
 
-    ret = nc_client_tls_ch_del_bind("127.0.0.1", 10010);
+    ret = nc_client_tls_ch_del_bind("127.0.0.1", TEST_PORT_2);
     assert_int_equal(ret, 0);
 
     nc_session_free(session, NULL);
@@ -400,7 +400,7 @@
     assert_int_equal(ret, 0);
 
     /* set call-home address and port */
-    ret = nc_server_config_add_ch_address_port(ctx, "ch_tls", "endpt", NC_TI_OPENSSL, "127.0.0.1", "10010", &test_state->tls_tree);
+    ret = nc_server_config_add_ch_address_port(ctx, "ch_tls", "endpt", NC_TI_OPENSSL, "127.0.0.1", TEST_PORT_2_STR, &test_state->tls_tree);
     assert_int_equal(ret, 0);
 
     /* set call-home server certificate */