Add RandomChoices with rarely().

Author: dweiss

randomizedtesting-jupiter/src/main/java/com/carrotsearch/randomizedtesting/jupiter/generators/RandomChoices.java

@@ -0,0 +1,11 @@
+package com.carrotsearch.randomizedtesting.jupiter.generators;
+
+import java.util.Random;
+
+/** Random selections of objects. */
+public final class RandomChoices {
+  /** Returns {@code true} rarely (~10% of calls). */
+  public static boolean rarely(Random r) {
+    return r.nextInt(100) >= 90;
+  }
+}