Fixed GH-8400: bug73837.phpt makes no sense

Replaces the indeed silly test with something that actually does the job,
albeit much slower.

ext/date/tests/bug73837.phpt

@@ -1,19 +1,23 @@
 --TEST--
 Bug #73837: Milliseconds in DateTime()
+--SKIPIF--
+<?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+?>
 --FILE--
 <?php
-$collect = [];
-
-for ( $i = 0; $i < 1000; $i++ )
-{
-    $a = new DateTime();
-    $key = "s" . $a->format( "u" );
-    $collect[$key] = true;
+$startTS = time();
+$prev_dt = new DateTime();
+while (time() < $startTS + 2) {
+    $dt = new DateTime();
+    if ($prev_dt > $dt) {
+        var_dump($prev_dt->format("Y-m-d H:i:s.u"));
+        var_dump($dt->format("Y-m-d H:i:s.u"));
+        break;
+    }
+    $prev_dt = $dt;
 }
-
-var_dump($n = count( $collect ));
-echo ( $n > 700 ) ? "microseconds differ\n" : "microseconds do not differ enough ($n)\n";
+echo "Finished\n";
 ?>
 --EXPECTF--
-int(%d)
-microseconds differ
+Finished