ARROW-11134: [CI][C++] Always run tests on Travis-CI
ARROW-11134: [CI][C++] Always run tests on Travis-CI

Also try to fix a compiler crash issue by limiting build parallelism.

Closes #9328 from pitrou/ARROW-11134-travis-run-tests

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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