Migrate reactor testing to Java (#5679)

Author: anuraaga

instrumentation/reactor/reactor-3.1/javaagent/src/test/groovy/ReactorCoreTest.groovy

@@ -0,0 +1,20 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.reactor;
+
+import io.opentelemetry.instrumentation.testing.junit.AgentInstrumentationExtension;
+import io.opentelemetry.instrumentation.testing.junit.InstrumentationExtension;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+class ReactorCoreTest extends AbstractReactorCoreTest {
+
+  @RegisterExtension
+  static final InstrumentationExtension testing = AgentInstrumentationExtension.create();
+
+  ReactorCoreTest() {
+    super(testing);
+  }
+}

instrumentation/reactor/reactor-3.1/javaagent/src/test/groovy/SubscriptionTest.groovy

@@ -0,0 +1,20 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.reactor;
+
+import io.opentelemetry.instrumentation.testing.junit.AgentInstrumentationExtension;
+import io.opentelemetry.instrumentation.testing.junit.InstrumentationExtension;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+class SubscriptionTest extends AbstractSubscriptionTest {
+
+  @RegisterExtension
+  static final InstrumentationExtension testing = AgentInstrumentationExtension.create();
+
+  SubscriptionTest() {
+    super(testing);
+  }
+}