test: Add in_ci utility test function

in_ci checks for environment variables set by a selection of CI systems
and returns true if the test appears to be running in CI. Additionally
it can test for specific CI systems, and returns true if the CI system
is listed.

Deduplicate existing environment checks for Travis and Jenkins.

Signed-off-by: Joe Groocock <me@frebib.net>

test/shell/t3.sh

@@ -2,7 +2,7 @@
 
 . "$(dirname "$0")/../util.sh"
 
-if env | grep -q -e '^TRAVIS=true$'; then
+if in_ci travis jenkins; then
 	# inotify doesn't seem to work properly on travis
 	echo "Travis and Jenkins give wonky results here, skipping test!"
 	exit