Update TestNG Version

Created by: jmini

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

To remove follwing Stacktrace:

[main] WARN  i.s.parser.util.DeserializationUtils - Error snake-parsing yaml content
io.swagger.parser.util.DeserializationUtils$SnakeException: Exception safe-checking yaml content  (maxDepth 2000)
	at io.swagger.parser.util.DeserializationUtils$CustomSnakeYamlConstructor.getSingleData(DeserializationUtils.java:300)
	at org.yaml.snakeyaml.Yaml.loadFromReader(Yaml.java:450)
	at org.yaml.snakeyaml.Yaml.load(Yaml.java:369)
	at io.swagger.parser.util.DeserializationUtils.readYamlTree(DeserializationUtils.java:137)
	at io.swagger.parser.Swagger20Parser.deserializeYaml(Swagger20Parser.java:83)
	at io.swagger.parser.Swagger20Parser.readWithInfo(Swagger20Parser.java:64)
	at io.swagger.parser.SwaggerParser.readWithInfo(SwaggerParser.java:32)
	at io.swagger.v3.parser.converter.SwaggerConverter.readLocation(SwaggerConverter.java:92)
	at io.swagger.parser.OpenAPIParser.readLocation(OpenAPIParser.java:19)
	at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:606)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:367)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)
Caused by: java.lang.IllegalAccessError: tried to access field org.yaml.snakeyaml.constructor.BaseConstructor.composer from class io.swagger.parser.util.DeserializationUtils$CustomSnakeYamlConstructor
	at io.swagger.parser.util.DeserializationUtils$CustomSnakeYamlConstructor.getSingleData(DeserializationUtils.java:279)
	... 11 common frames omitted

That is due to an outdated version of org.yaml:snakeyaml, we need to update TestNG.

Before the change, our TestNG version 6.9.6 (very old) requires version 1.15 of org.yaml:snakeyaml as compile dependency.

I took the opportunity of this PR to also remove the JUnit stuff that was configured and used in some of our tests.