Add dependency on opendmk_jmxremote_optional_jar for performance builds

Although the jmxremote_optional JAR has been added to the build agents
on the Bamboo CI server for the latest JDK 8 installation, that
solution is brittle since it has to be manually installed in every new
JDK installation. In addition, this approach will not work with JDK 9+
since the "Extension Mechanism" has been removed beginning with JDK 9.

https://docs.oracle.com/javase/10/migrate/toc.htm#JSMIG-GUID-2C896CA8-927C-4381-A737-B1D81D964B7B

This commit addresses this issue by adding the following dependency to
spring-context.

// Substitute for "javax.management:jmxremote_optional:1.0.1_04" which
// is not available on Maven Central
testRuntime("org.glassfish.external:opendmk_jmxremote_optional_jar:1.0-b01-ea")

With this change, the Spring PERFORMANCE builds now execute on JDK 8,
9, and 11.

See gh-22757

spring-context/spring-context.gradle

@@ -28,6 +28,9 @@ dependencies {
 	testCompile("org.codehaus.groovy:groovy-xml:${groovyVersion}")
 	testCompile("org.apache.commons:commons-pool2:2.6.0")
 	testCompile("javax.inject:javax.inject-tck:1")
+	// Substitute for "javax.management:jmxremote_optional:1.0.1_04" which
+	// is not available on Maven Central
+	testRuntime("org.glassfish.external:opendmk_jmxremote_optional_jar:1.0-b01-ea")
 	testCompile("org.awaitility:awaitility:3.1.3")
 	testRuntime("javax.xml.bind:jaxb-api:2.3.1")
 	testRuntime("org.glassfish:javax.el:3.0.1-b08")