Configure TestJdk9 for Develocity

Prevously, the tests that are run in the `TestJdk9` configuration were
not captured by Develocity: the results of these tests would not appear
in the Develocity test report and dashboard, and failed tests would not
be retried.

Because TestJdk9 is a custom configuration, Develocity cannot configure
it automatically and we need to manually add the Develocity test
settings in that configuration. This is done through an auto plugin that
is enabled only on projects where both the Develocity and Jdk9 plugins
are enabled.

project/PekkoDevelocityPlugin.scala

@@ -22,7 +22,7 @@ import com.gradle.develocity.agent.sbt.DevelocityPlugin.autoImport.{
   ProjectId,
   Publishing
 }
-import sbt.{ url, AutoPlugin, Def, PluginTrigger, Plugins, Setting }
+import sbt.{ inConfig, url, AutoPlugin, Def, PluginTrigger, Plugins, Setting }
 import sbt.Keys.insideCI
 
 object PekkoDevelocityPlugin extends AutoPlugin {
@@ -63,3 +63,15 @@ object PekkoDevelocityPlugin extends AutoPlugin {
       } else apacheDevelocityConfiguration
     })
 }
+
+/**
+ * An AutoPlugin to add Develocity test configuration to the TestJdk9 configuration.
+ */
+object PekkoDevelocityJdk9TestSettingsPlugin extends AutoPlugin {
+  override lazy val trigger: PluginTrigger = allRequirements
+  override lazy val requires: Plugins = DevelocityPlugin && Jdk9
+
+  // See https://docs.gradle.com/develocity/sbt-plugin/#capturing_test_data_in_a_custom_configuration
+  override lazy val projectSettings =
+    inConfig(Jdk9.TestJdk9)(DevelocityPlugin.testSettings)
+}