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/test-gotest.sh

@@ -8,7 +8,7 @@ cd "${ROOT}"
 . test/util.sh
 
 # travis is slow for some reason
-if env | grep -q -e '^TRAVIS=true$'; then
+if in_ci travis; then
 	export GO_TEST_TIMEOUT_SCALE=3
 fi