Update java8 tests to run against java11 (GoogleCloudPlatform#2029)

* Update java8 tests to run against java11

* fix pom

* add allowed java versions

* remove sample

Author: averikitsch

.kokoro/java8/common.cfg

@@ -32,7 +32,7 @@ action {
 # Set the JAVA VERSION env var.
 env_vars: {
     key: "JAVA_VERSION"
-    value: "1.8"
+    value: "1.8, 11"
 }
 
 # Configure the docker image for kokoro-trampoline.

.kokoro/tests/run_tests.sh

@@ -110,7 +110,8 @@ for file in **/pom.xml; do
 
     # Fail the tests if no Java version was found.
     POM_JAVA=$(grep -oP '(?<=<maven.compiler.target>).*?(?=</maven.compiler.target>)' pom.xml)
-    if [[ "$POM_JAVA" = "" ]]; then
+    ALLOWED_VERSIONS=("1.8" "11")
+    if [[ "$POM_JAVA" = "" ]] || [[ !  "${ALLOWED_VERSIONS[@]}" =~ "${POM_JAVA}" ]]; then
         RTN=1
         echo -e "\n Testing failed: Unable to determine Java version. Please set in pom:"
         echo -e "\n<properties>"