Avoid to run rspec tests directly from Gradle script

[andsel]

Running rubyTests with JDK 21 exposes the error:

            should end at the number of generated events (FAILED - 3)
    [2024-02-01T14:18:25,583][ERROR][logstash.javapipeline    ] Pipeline error {:pipeline_id=>"main", :exception=>#<ArgumentError: wrong number of arguments (given 1, expected 0)>, 
:backtrace=>[
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/config/lir_serializer.rb:47:in `vertices'", 
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/config/lir_serializer.rb:36:in `serialize'", 
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/config/lir_serializer.rb:27:in `serialize'", 
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/java_pipeline.rb:268:in `start_workers'", 
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/java_pipeline.rb:194:in `run'", 
"/Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/java_pipeline.rb:146:in `block in start'"
], 
"pipeline.sources"=>[], :thread=>"#<Thread:0x5e2f743e /Users/andrea/workspace/logstash_andsel/logstash-core/lib/logstash/java_pipeline.rb:134 run>"}

To reproduce consider the PR #15719, after set system java to a JDK 21, run with:

SPEC_OPTS="-fd -P logstash-core/spec/logstash/pipeline_reporter_spec.rb" ./gradlew :logstash-core:rubyTests --tests org.logstash.RSpecTests --rerun-tasks

The problem happens at

logstash/logstash-core/lib/logstash/config/lir_serializer.rb

Line 47 in 8ac5518

graph.getVertices.map {|v| vertex(v) }.compact

Where getVertices returns a LinkedHashSet.
This error was isolated in JRuby issue jruby/jruby#8061, however originates from the fact that Logstash's rubyTests Gradle tasks, executes a JRuby script engine within the Gradle process itself:

• logstash/logstash-core/build.gradle

  Line 146 in 8ac5518

  include '/org/logstash/RSpecTests.class'

• logstash/logstash-core/src/test/java/org/logstash/RSpecTests.java

  Line 67 in 8ac5518

  final IRubyObject result = RubyUtil.RUBY.executeScript(

This issue asks to switch the execution of rubyTest into a separated process, outside of the Gradle daemon, so that the test environment could be more close to what the real environment is, without interference of Gradle internals itself.

[andsel]

/cc @roaksoax @jsvd