Dúvida sobre o carremento de módulos do RESTEASY no Wildfly

Estou tentando criar um teste simples com o Client client = ClientBuilder.newClient();

javax.ws.rs.client.Client;
javax.ws.rs.client.ClientBuilder;

Porém recebo o seguinte erro:

java.lang.ClassNotFoundException: org.glassfish.jersey.client.JerseyClientBuilder
java.lang.RuntimeException
at javax.ws.rs.client.ClientBuilder.newBuilder(ClientBuilder.java:103)
at javax.ws.rs.client.ClientBuilder.newClient(ClientBuilder.java:114)
at com.study.restful.test.ProductResourceTest.setUp(ProductResourceTest.java:34)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.apache.maven.surefire.junit4.JUnit4TestSet.execute(JUnit4TestSet.java:53)
at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:123)
at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:104)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:164)
at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:110)
at org.apache.maven.surefire.booter.SurefireStarter.invokeProvider(SurefireStarter.java:175)
at org.apache.maven.surefire.booter.SurefireStarter.runSuitesInProcessWhenForked(SurefireStarter.java:107)
at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:68)
Caused by: java.lang.ClassNotFoundException: org.glassfish.jersey.client.JerseyClientBuilder
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at javax.ws.rs.client.FactoryFinder.newInstance(FactoryFinder.java:113)
at javax.ws.rs.client.FactoryFinder.find(FactoryFinder.java:206)
at javax.ws.rs.client.ClientBuilder.newBuilder(ClientBuilder.java:86)
… 32 more

Pesquisando na internet, ao adicionar a seguinte dependência no meu POM eu resolvo meu problema

<dependency>
    <groupId>org.jboss.resteasy</groupId>
     <artifactId>resteasy-client</artifactId>
     <version>3.1.1.Final</version>
 </dependency>

Porém essa mesma dependência JÁ EXISTE nos módulos do Wildfly, na pasta: /wildfly-10.1.0.Final/modules/system/layers/base/org/jboss/resteasy/resteasy-jaxrs.

Por que eu preciso adicionar a dependência ao pom.xml ? O Wildfly não deveria carregar isso pra mim ?

Quando você baixa e roda o wildfly/jboss, ele estará sendo executado no modo básico.
Eu recomendo que você vá até o local onde instalou o wildfly e altere o standalone.xml para standalone-old.xml e renomeie o arquivo standalone-full.xml para standalone.xml. Talvez isso resolva.