Docs: Replace deprecated pluginList with Arrays.asList (#24270)

ESIntegTestCase#pluginList was remove removed in ES 5.0. We are using Arrays.asList instead.

docs/reference/testing/testing-framework.asciidoc

@@ -171,7 +171,7 @@ As elasticsearch is using JUnit 4, using the `@Before` and `@After` annotations
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 @Override
 protected Collection<Class<? extends Plugin>> nodePlugins() {
-  return pluginList(CustomSuggesterPlugin.class);
+  return Arrays.asList(CustomSuggesterPlugin.class);
 }
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