Remove a check from setOldestVersion

Internal.h

@@ -427,6 +427,7 @@ struct ReferenceImpl {
   }
 
   void setOldestVersion(int64_t oldestVersion) {
+    assert(oldestVersion >= oldestVersion);
     this->oldestVersion = oldestVersion;
   }
 
@@ -569,6 +570,11 @@ template <class ConflictSetImpl> struct TestDriver {
       CALLGRIND_STOP_INSTRUMENTATION;
 #endif
       refImpl.addWrites(writes, numPointWrites + numRangeWrites);
+
+      oldestVersion = std::max<int64_t>(writeVersion - arbitrary.bounded(10),
+                                        oldestVersion);
+      cs.setOldestVersion(oldestVersion);
+      refImpl.setOldestVersion(oldestVersion);
     }
     {
       int numPointReads = arbitrary.bounded(100);
@@ -659,9 +665,6 @@ template <class ConflictSetImpl> struct TestDriver {
         }
       }
     }
-    oldestVersion += arbitrary.bounded(2);
-    cs.setOldestVersion(oldestVersion);
-    refImpl.setOldestVersion(oldestVersion);
     return false;
   }
 };