chore: add unit tests for HttpBigQueryRpc (#12067)

Add full unit test coverage for HttpBigQueryRpc for easy validation of
refactor changes for the observability project [see fast follow
PR](#12066).

Author: ldetmer

java-bigquery/google-cloud-bigquery/pom.xml

@@ -216,6 +216,11 @@
       <artifactId>opentelemetry-sdk-trace</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>io.opentelemetry</groupId>
+      <artifactId>opentelemetry-sdk-testing</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>