chore(util) do not remove test servroot with TEST_NGINX_RANDOMIZE=1

The servroot should only be cleaned when we are sure it will be
overwritten anyway. This allows observing its contents for debugging
purposes while running randomized test on the whole suite.

util/test.sh

@@ -140,6 +140,9 @@ fi
 if [[ "$TEST_NGINX_RANDOMIZE" == 1 ]]; then
     prove_opts="-j$(n_jobs)"
     echo "TEST_NGINX_RANDOMIZE=$TEST_NGINX_RANDOMIZE ($prove_opts)"
+
+else
+    rm -rf $TEST_NGINX_SERVROOT
 fi
 
 echo
@@ -155,8 +158,6 @@ elif [ ! -x "$(command -v ldd)" ]; then
     ldd $TEST_NGINX_BINARY | grep wasm
 fi
 
-rm -rf $TEST_NGINX_SERVROOT
-
 echo
 exec prove -r $prove_opts $@