Ensure only plugin REST tests are run for plugins

[jakelandis]

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 #52114
fixes #53183

---

Note - this is how I tested

# from elasticsearch directory
./gradlew :build-tools:publishToMavenLocal

./gradlew :rest-api-spec:publishToMavenLocal :test:framework:publishToMavenLocal :server:publishToMavenLocal :libs:elasticsearch-core:publishToMavenLocal :libs:elasticsearch-x-content:publishToMavenLocal :libs:elasticsearch-cli:publishToMavenLocal :libs:elasticsearch-nio:publishToMavenLocal :libs:elasticsearch-geo:publishToMavenLocal :libs:elasticsearch-secure-sm:publishToMavenLocal :distribution:archives:integ-test-zip:publishToMavenLocal :client:rest:publishToMavenLocal :client:sniffer:publishToMavenLocal :test:logger-usage:publishToMavenLocal

We can use @spinscale 's cookie cutter to help create a test project

# requires python3
pip3 install cookiecutter
cookiecutter gh:spinscale/cookiecutter-elasticsearch-ingest-processor
# Add the master version when it asks: elasticsearch_version [7.6.1]: 8.0.0-SNAPSHOT
cd ingest-awesome
# Add to the build.gradle
repositories {
   mavenLocal()
}
./gradlew check 

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Plugins)

[mark-vieira]

One minor comment on naming, otherwise LGTM.