fix gradle test after switching to testNG

Author: frantuma

modules/swagger-gradle-plugin/src/test/java/io/swagger/v3/plugins/gradle/SwaggerResolveTest.java

@@ -54,8 +54,8 @@ public void setup() throws IOException {
 
     @Test
     public void testSwaggerResolveTask() throws IOException {
-        outputDir = testProjectDir.getRoot().toString() + "/target";
-        outputFile = testProjectDir.getRoot().toString() + "/testAPI.json";
+        outputDir = testProjectDir.toString() + "/target";
+        outputFile = testProjectDir.toString() + "/testAPI.json";
         //outputDir = "/tmp/a/target";
         String resolveTask = "resolve";
 
@@ -127,8 +127,8 @@ public void testSwaggerResolveTask() throws IOException {
 
     @Test
     public void testSwaggerResolveWithOAS31OptionTask() throws IOException {
-        outputDir = testProjectDir.getRoot().toString() + "/target";
-        outputFile = testProjectDir.getRoot().toString() + "/testAPI31.json";
+        outputDir = testProjectDir.toString() + "/target";
+        outputFile = testProjectDir.toString() + "/testAPI31.json";
         //outputDir = "/tmp/a/target";
         String resolveTask = "resolve";