Use port free even under macOS 12 port for running httpbin

The default httpbin listening port 5000 is used by Monterrey (macOS 12)
Control Center for its own needs, so use another port to avoid conflicts
with it and the resulting failures when running wxWebRequest unit tests
under this macOS version.

build/tools/httpbin.sh

@@ -66,8 +66,8 @@ httpbin_launch() {
     echo "Installing using `python3 -m pip --version`"
 
     python3 -m pip install $pip_explicit_deps httpbin --user
-    python3 -m httpbin.core 2>&1 >httpbin.log &
-    WX_TEST_WEBREQUEST_URL="http://localhost:5000"
+    python3 -m httpbin.core --port 50500 2>&1 >httpbin.log &
+    WX_TEST_WEBREQUEST_URL="http://localhost:50500"
 }
 
 httpbin_show_log() {