Ensure only plugin REST tests are run for plugins

This commit fixes ensures that for external builds
(e.g. plugin development) that the REST tests that are
copied are properly filtered to only include the API
by default.

The code prior to this change resulted in including both
the API and tests since the copy.include resulted as an
empty list by default since the stream is empty unless
explictly configured.

related elastic#52114

buildSrc/src/main/java/org/elasticsearch/gradle/test/rest/CopyRestApiTask.java

@@ -132,7 +132,11 @@ void copy() {
             project.copy(c -> {
                 c.from(project.zipTree(coreConfig.getSingleFile()));
                 c.into(getTestSourceSet().getOutput().getResourcesDir()); // this ends up as the same dir as outputDir
-                c.include(includeCore.get().stream().map(prefix -> COPY_TO + "/" + prefix + "*/**").collect(Collectors.toList()));
+                if (includeCore.get().isEmpty()) {
+                    c.include(COPY_TO + "/**");
+                } else {
+                    c.include(includeCore.get().stream().map(prefix -> COPY_TO + "/" + prefix + "*/**").collect(Collectors.toList()));
+                }
             });
         }
         // only copy x-pack specs if explicitly instructed