Update TestNG6 DTD resolution during tests (#11206)

# What Does This Do

- In TestNG instrumentation tests related to XML-defined test suites, switches the DTD URL in `<!DOCTYPE suite SYSTEM …>` from `https://testng.org/testng-1.0.dtd` to `http://testng.org/testng-1.0.dtd` so the XML parser relies on the classpath-bundled DTD.


# Motivation

TestNG's XML parsers will try to resolve and validate the DTD definition when running the test suites. This is a source of flakiness for the instrumentation tests as the resolution depends on network calls.

# Additional Notes

test-environment-trigger: skip

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Jira ticket: [PROJ-IDENT]

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Co-authored-by: daniel.mohedano <daniel.mohedano@datadoghq.com>

Author: daniel-mohedano

dd-java-agent/instrumentation/testng/testng-6.4/src/test/resources/test-successful-test-cases-in-TESTS-parallel-mode-not-all-test-methods-included/suite.xml

@@ -1,4 +1,4 @@
-<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd" >
+<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
 <suite name="API Test Suite" parallel="tests" configfailurepolicy="continue">
   <test name="Test A">
     <classes>

dd-java-agent/instrumentation/testng/testng-6.4/src/test/resources/test-successful-test-cases-in-TESTS-parallel-mode-same-test-case/suite.xml

@@ -1,4 +1,4 @@
-<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd" >
+<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
 <suite name="API Test Suite" parallel="tests" configfailurepolicy="continue">
   <test name="Test A">
     <classes>

dd-java-agent/instrumentation/testng/testng-6.4/src/test/resources/test-successful-test-cases-in-TESTS-parallel-mode/suite.xml

@@ -1,4 +1,4 @@
-<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd" >
+<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
 <suite name="API Test Suite" parallel="tests" configfailurepolicy="continue">
   <test name="Test A">
     <classes>