Improve substrate-median tests

The use of a dedicated test module ensures the API doesn't hide anything which
needs to be public. There's also now explicit tests for when the median is the
popped value.

substrate/median/README.md

@@ -1,3 +1,8 @@
 # Substrate Median
 
 An efficient implementation of a median algorithm within a Substrate runtime.
+
+For more information, please see the
+[documentation](
+  https://docs.rs/substrate-median/latest/substrate_median/trait.Median.html
+).