Add overlay tests

test/spec/fixtures/repos/jdk-overlay-java-8/.jdk-overlay/extra.txt

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+extra.txt contents

test/spec/fixtures/repos/jdk-overlay-java-8/system.properties

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+java.runtime.version=8

test/spec/fixtures/repos/jdk-overlay/.jdk-overlay/extra.txt

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+extra.txt contents

test/spec/fixtures/repos/jdk-overlay/system.properties

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+java.runtime.version=21

test/spec/fixtures/repos/jvmoptions/jvmoptions.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_21" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/src/main/resources" type="java-resource" />
+    </content>
+    <orderEntry type="jdk" jdkName="21" jdkType="JavaSDK" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

test/spec/overlay_spec.rb

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+# frozen_string_literal: true
+
+require_relative 'spec_helper'
+
+RSpec.describe 'JDK overlay' do
+  it 'is applied correctly over OpenJDK 8' do
+    app = Hatchet::Runner.new('jdk-overlay-java-8')
+
+    app.deploy do
+      expect(app.output).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
+        remote: -----> JVM Common app detected        
+        remote: -----> Installing OpenJDK 8... done        
+      REGEX
+
+      expect(app.run('cat .jdk/extra.txt')).to eq("extra.txt contents\n")
+      expect(app.run('md5sum .jdk/jre/lib/security/cacerts')).to start_with('86700d98c9b3aaf30b40fabcc12c75fe')
+    end
+  end
+
+  it 'is applied correctly over OpenJDK >8 (21)' do
+    app = Hatchet::Runner.new('jdk-overlay')
+
+    app.deploy do
+      expect(app.output).to match(Regexp.new(<<~REGEX, Regexp::MULTILINE))
+        remote: -----> JVM Common app detected        
+        remote: -----> Installing OpenJDK 21... done        
+      REGEX
+
+      expect(app.run('cat .jdk/extra.txt')).to eq("extra.txt contents\n")
+      expect(app.run('md5sum .jdk/lib/security/cacerts')).to start_with('86700d98c9b3aaf30b40fabcc12c75fe')
+    end
+  end
+end