Provide explicit locale in GridTest (#8525)

Fixes #8524

server/src/test/java/com/vaadin/tests/server/component/grid/GridTest.java

@@ -5,11 +5,13 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 
 import java.text.DecimalFormat;
+import java.text.DecimalFormatSymbols;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Optional;
 import java.util.Random;
 import java.util.Set;
@@ -504,7 +506,8 @@ public class GridTest {
         Grid<Person> grid = new Grid<>(Person.class);
 
         grid.removeColumn("born");
-        grid.addColumn("born", new NumberRenderer(new DecimalFormat("#,###")));
+        grid.addColumn("born", new NumberRenderer(new DecimalFormat("#,###",
+                DecimalFormatSymbols.getInstance(Locale.US))));
 
         Person person = new Person("Name", 2017);