Simplify backend mock access for unittests (#1062)

src/data/BackendInterface.cpp

@@ -270,6 +270,19 @@ BackendInterface::updateRange(uint32_t newMax)
     }
 }
 
+void
+BackendInterface::setRange(uint32_t min, uint32_t max, bool force)
+{
+    std::scoped_lock const lck(rngMtx_);
+
+    if (!force) {
+        ASSERT(min <= max, "Range min must be less than or equal to max");
+        ASSERT(not range.has_value(), "Range was already set");
+    }
+
+    range = {min, max};
+}
+
 LedgerPage
 BackendInterface::fetchLedgerPage(
     std::optional<ripple::uint256> const& cursor,