CORDA-3644: Add Corda-Testing tag to test artifacts' MANIFEST.MF. (#6032)

testing/test-db/build.gradle

@@ -16,6 +16,11 @@ dependencies {
 
 jar {
     baseName 'corda-test-db'
+    manifest {
+        // This JAR is part of Corda's testing framework.
+        // Driver will not include it as part of an out-of-process node.
+        attributes('Corda-Testing': true)
+    }
 }
 
 publish {