Better tests for php-nextgen (#16569)

* better tests for php nextgen

* add new files

samples/client/echo_api/php-nextgen/.openapi-generator/FILES

@@ -0,0 +1,47 @@
+.gitignore
+.php-cs-fixer.dist.php
+.phplint.yml
+.travis.yml
+README.md
+composer.json
+docs/Api/AuthApi.md
+docs/Api/BodyApi.md
+docs/Api/FormApi.md
+docs/Api/HeaderApi.md
+docs/Api/PathApi.md
+docs/Api/QueryApi.md
+docs/Model/Bird.md
+docs/Model/Category.md
+docs/Model/DataQuery.md
+docs/Model/DefaultValue.md
+docs/Model/NumberPropertiesOnly.md
+docs/Model/Pet.md
+docs/Model/Query.md
+docs/Model/StringEnumRef.md
+docs/Model/Tag.md
+docs/Model/TestQueryStyleDeepObjectExplodeTrueObjectAllOfQueryObjectParameter.md
+docs/Model/TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter.md
+git_push.sh
+phpunit.xml.dist
+src/Api/AuthApi.php
+src/Api/BodyApi.php
+src/Api/FormApi.php
+src/Api/HeaderApi.php
+src/Api/PathApi.php
+src/Api/QueryApi.php
+src/ApiException.php
+src/Configuration.php
+src/HeaderSelector.php
+src/Model/Bird.php
+src/Model/Category.php
+src/Model/DataQuery.php
+src/Model/DefaultValue.php
+src/Model/ModelInterface.php
+src/Model/NumberPropertiesOnly.php
+src/Model/Pet.php
+src/Model/Query.php
+src/Model/StringEnumRef.php
+src/Model/Tag.php
+src/Model/TestQueryStyleDeepObjectExplodeTrueObjectAllOfQueryObjectParameter.php
+src/Model/TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter.php
+src/ObjectSerializer.php