Fix FormDataMapTest anonymous FormValue missing undertow 2.2.x methods

Add getCharset(), getFileItem(), isFileItem(), and isBigField() stubs
to the anonymous FormData.FormValue in addInMemoryFileValue(). These
abstract methods were added in undertow 2.2.x and caused compilation
failure when the latestDepForkedTest resolved the latest 2.2.x release.

Author: jandro996

dd-java-agent/instrumentation/undertow/undertow-2.0/src/test/java/datadog/trace/instrumentation/undertow/FormDataMapTest.java

@@ -132,6 +132,27 @@ public File getFile() {
           public HeaderMap getHeaders() {
             return null;
           }
+
+          // Methods added in undertow 2.2.x
+          @Override
+          public String getCharset() {
+            return null;
+          }
+
+          @Override
+          public FormData.FileItem getFileItem() {
+            return null;
+          }
+
+          @Override
+          public boolean isFileItem() {
+            return false;
+          }
+
+          @Override
+          public boolean isBigField() {
+            return false;
+          }
         };
 
     Deque<FormData.FormValue> deque = new ArrayDeque<>();