Weaken random date formatter test assertion

`ESTestCase#testRandomDateFormatterPattern` previously asserted that
round tripping `millis -> text -> millis` wouldn't lose any precision.
But some date formats don't include the time of day so, of course, this
could lose precision. This replaces that with an assertion that
`text -> millis -> text` doesn't lose precision. Which should be true
for any sane date format. Really, we're just trying to make sure that
the random date formats that we return are *fairly* sane.

test/framework/src/test/java/org/elasticsearch/test/test/ESTestCaseTests.java

@@ -204,6 +204,17 @@ public class ESTestCaseTests extends ESTestCase {
 
     public void testRandomDateFormatterPattern() {
         DateFormatter formatter = DateFormatter.forPattern(randomDateFormatterPattern());
-        assertThat(formatter.parseMillis(formatter.formatMillis(0)), equalTo(0L));
+        /*
+         * Make sure it doesn't crash trying to format some dates and
+         * that round tripping through millis doesn't lose any information.
+         * Interestingly, round tripping through a string *can* lose
+         * information because not all date formats spit out milliseconds.
+         * Hell, not all of them spit out the time of day at all!
+         * But going from text back to millis back to text should
+         * be fine!
+         */
+        String formatted = formatter.formatMillis(randomLongBetween(0, 2_000_000_000_000L));
+        String formattedAgain = formatter.formatMillis(formatter.parseMillis(formatted));
+        assertThat(formattedAgain, equalTo(formatted));
     }
 }