Default to Java 8 in run-tests.sh

Author: jyemin

.evergreen/run-tests.sh

@@ -19,7 +19,7 @@ set -o errexit  # Exit the script with error if any of the commands fail
 AUTH=${AUTH:-noauth}
 SSL=${SSL:-nossl}
 MONGODB_URI=${MONGODB_URI:-}
-JDK=${JDK:-jdk}
+JDK=${JDK:-jdk8}
 TOPOLOGY=${TOPOLOGY:-server}
 COMPRESSOR=${COMPRESSOR:-}
 SLOW_TESTS_ONLY=${SLOW_TESTS_ONLY:-false}