Merge #19613: travis: Re-enable s390x

fae6561 travis: Re-enable s390x (MarcoFalke)

Pull request description:

  According to travis, the issue has been solved. Quote

  > I would like to confirm that we have resolved this issue and most of our users are reported that this issue has been resolved on their end as well. Could you please re-check and see if that still exists for you?

ACKs for top commit:
  theStack:
    ACK bitcoin/bitcoin@fae6561

Tree-SHA512: cf42f96d25474a9dcf0817a049e30e29714731d708f73c40a3042b0c70a71ff08f07dd96a89f0dcd5a50a63a355cf30b3511172a32b8af7d5a2e13ad222a4b49

Author: MarcoFalke

.travis.yml

@@ -72,14 +72,12 @@ jobs:
         FILE_ENV="./ci/test/00_setup_env_arm.sh"
         QEMU_USER_CMD=""
 
-# s390 build was disabled temporarily because of disk space issues on the Travis VM
-#
-#    - stage: test
-#      name: 'S390x  [GOAL: install]  [buster]  [unit tests, functional tests]'
-#      arch: s390x  # Can disable QEMU_USER_CMD and run the tests natively without qemu
-#      env: >-
-#        FILE_ENV="./ci/test/00_setup_env_s390x.sh"
-#        QEMU_USER_CMD=""
+    - stage: test
+      name: 'S390x  [GOAL: install]  [buster]  [unit tests, functional tests]'
+      arch: s390x  # Can disable QEMU_USER_CMD and run the tests natively without qemu
+      env: >-
+        FILE_ENV="./ci/test/00_setup_env_s390x.sh"
+        QEMU_USER_CMD=""
 
     - stage: test
       name: 'Win64  [GOAL: deploy]  [unit tests, no gui, no functional tests]'