Increase sleep in distributable tests to handle slow Mac first start

On MacOS, in CI, it can take a couple of extra seconds for the server to
start and certificates to be generated. Not a big problem in practice,
but makes the integration tests flaky, so this should stabilise that.

Author: pimterry

test/distributables-test/unix.sh

@@ -11,7 +11,7 @@ echo "\nStarting server..."
 ./httptoolkit-server/bin/httptoolkit-server start &
 SERVER_PID=$!
 
-sleep 5
+sleep 10
 
 echo "\nTesting server..."