Changed toPrecision test to use toFixed

Despite the bad name, this is to agree with the Java ibex_gui
  code which uses fixed precision (N decimal places, rather
  than N significant figures).

app/components/InstrumentPage.test.ts

@@ -49,7 +49,7 @@ test("toPrecision truncates block if it has precision", () => {
     precision: precision,
   };
   expect(toPrecision(aBlock, originalValue)).toBe(
-    originalValue.toPrecision(precision),
+    originalValue.toFixed(precision),
   );
 });