build: move testbench-rpc dependency to test scope

Author: javier-godoy

pom.xml

@@ -146,6 +146,7 @@
              <groupId>com.flowingcode.vaadin.test</groupId>
              <artifactId>testbench-rpc</artifactId>
              <version>1.4.0</version>
+             <scope>test</scope>
          </dependency>
         <dependency>
             <groupId>org.hamcrest</groupId>