[SPARK-21982] Set locale to US

## What changes were proposed in this pull request?

In UtilsSuite Locale was set by default to US, but at the moment of using format function it wasn't, taking by default JVM locale which could be different than US making this test fail.

## How was this patch tested?
Unit test (UtilsSuite)

Author: German Schiavon <[email protected]>

Closes apache#19205 from Gschiavon/fix/test-locale.

Author: Gschiavon

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -1193,16 +1193,17 @@ private[spark] object Utils extends Logging {
     val second = 1000
     val minute = 60 * second
     val hour = 60 * minute
+    val locale = Locale.US
 
     ms match {
       case t if t < second =>
-        "%d ms".format(t)
+        "%d ms".formatLocal(locale, t)
       case t if t < minute =>
-        "%.1f s".format(t.toFloat / second)
+        "%.1f s".formatLocal(locale, t.toFloat / second)
       case t if t < hour =>
-        "%.1f m".format(t.toFloat / minute)
+        "%.1f m".formatLocal(locale, t.toFloat / minute)
       case t =>
-        "%.2f h".format(t.toFloat / hour)
+        "%.2f h".formatLocal(locale, t.toFloat / hour)
     }
   }