* Use only the free pipelines arm64/ppc64le/s390x without credits on Travis.
See <https://docs.travis-ci.com/user/billing-overview/#partner-queue-solution>.
* Add ppc64le case.
* Update the gcc to gcc-11.
* Remove unused logic.
* Add the `tool/travis_wait.sh` to avoid a command timeout.
* Run the tests skipping some failing tests.
* Set `TEST_ALL_SKIPPED_TESTS` env to skip the tests.
* Run the tests separately returning ok status.
The tests are executed if `TEST_ALL_SKIPPED_TESTS` env is set or
`TEST_ALL_SEPARATED_TESTS` env is set.
* Add `tool/ci_functions.sh` to manage the functions used in CI.
* Add arm64 to allow_failures due to the following issue.
An arm64 job sometimes does not start right now.
https://travis-ci.community/t/11629
