In-memory tests for searching changes

Use InMemoryModule to inject everything needed to manually insert
commits into a repo and changes into a database, then query the
results.

Change-Id: I22f6d663039039dd4d3f2189e587dbd22f11252b

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeJson.java

@@ -924,7 +924,7 @@ public class ChangeJson {
     Boolean mergeable;
 
     String _sortkey;
-    int _number;
+    public int _number;
 
     AccountInfo owner;