travis: increase open file limits (#18155)

.travis.yml

@@ -29,6 +29,14 @@ matrix:
     - os: osx
       go: 1.11.x
       script:
+        - echo "Increase the maximum number of open file descriptors on macOS"
+        - NOFILE=20480
+        - sudo sysctl -w kern.maxfiles=$NOFILE
+        - sudo sysctl -w kern.maxfilesperproc=$NOFILE
+        - sudo launchctl limit maxfiles $NOFILE $NOFILE
+        - sudo launchctl limit maxfiles
+        - ulimit -S -n $NOFILE
+        - ulimit -n
         - unset -f cd # workaround for https://github.com/travis-ci/travis-ci/issues/8703
         - go run build/ci.go install
         - go run build/ci.go test -coverage $TEST_PACKAGES