ci: re-enable gui tests for s390x

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

real	51m37.809s
```

Author: fanquake

ci/test/00_setup_env_s390x.sh

@@ -23,4 +23,4 @@ export TEST_RUNNER_ENV="LC_ALL=C"
 export TEST_RUNNER_EXTRA="--exclude feature_init,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"  # GUI tests disabled for now, see https://github.com/bitcoin/bitcoin/issues/23730
+export BITCOIN_CONFIG="--enable-reduce-exports"