Converter tests:

Change test data files extension to "java"

j2k/tests/testData/ast/issues/kt-879.java

@@ -0,0 +1,17 @@
+//file
+class Test {
+    public static int getInt(int i) {
+        switch (i) {
+            case 0:
+                return 0;
+            case 1:
+                return 1;
+            case 2:
+                return 2;
+            case 3:
+                return 3;
+            default:
+                return -1;
+        }
+    }
+}