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_tls.c b/tests/test_tls.c
index 542f7bd..b02bc30 100644
--- a/tests/test_tls.c
+++ b/tests/test_tls.c
@@ -85,7 +85,7 @@
     assert_int_equal(ret, 0);
 
     pthread_barrier_wait(&state->barrier);
-    session = nc_connect_tls("127.0.0.1", 10005, NULL);
+    session = nc_connect_tls("127.0.0.1", TEST_PORT, NULL);
     assert_non_null(session);
 
     nc_session_free(session, NULL);
@@ -138,7 +138,7 @@
     assert_int_equal(ret, 0);
 
     /* create new address and port data */
-    ret = nc_server_config_add_address_port(ctx, "endpt", NC_TI_OPENSSL, "127.0.0.1", 10005, &tree);
+    ret = nc_server_config_add_address_port(ctx, "endpt", NC_TI_OPENSSL, "127.0.0.1", TEST_PORT, &tree);
     assert_int_equal(ret, 0);
 
     /* create new server certificate data */