Merge bitcoin#28014: ci: re-enable gui tests for s390x

9be4565 ci: re-enable gui tests for s390x (fanquake)

Pull request description:

  These work for me now. If they still don't work in other setups, maybe we can better document the issues.

  ```bash
  time FILE_ENV="./ci/test/00_setup_env_s390x.sh" ./ci/test_run_all.sh
  ...
  Running tests: coins_tests from test/coins_tests.cpp
  PASS: qt/test/test_bitcoin-qt
  Running tests: coinstatsindex_tests from test/coinstatsindex_tests.cpp
  ...
  Stop and remove CI container by ID
  + docker container kill 617bef8accb87530e5fbb03ff07b3b9f0aa9e3030d4da424c9612d153ab98dbf
  617bef8accb87530e5fbb03ff07b3b9f0aa9e3030d4da424c9612d153ab98dbf

  real51m37.809s
  ```

ACKs for top commit:
  MarcoFalke:
    lgtm ACK 9be4565 , didn't test
  hebasto:
    ACK 9be4565, tested on Ubuntu 23.04.

Tree-SHA512: 2ab708013fab7f9bf06cfc106cdffd3b7733cc7b2fe5b2122e915895735452c9b4a70ccc5a8435b28655369d54820fcf62ecb89791d5ee2e349724857e944796

Author: fanquake

ci/test/00_setup_env_s390x.sh

@@ -22,4 +22,4 @@ export RUN_UNIT_TESTS=true
 export TEST_RUNNER_EXTRA="--exclude rpc_bind,feature_bind_extra"  # Excluded for now, see https://github.com/bitcoin/bitcoin/issues/17765#issuecomment-602068547
 export RUN_FUNCTIONAL_TESTS=true
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests --with-boost-process"  # GUI tests disabled for now, see https://github.com/bitcoin/bitcoin/issues/23730
+export BITCOIN_CONFIG="--enable-reduce-exports --with-boost-process"