Prefer const in test files (#1117)

* Removed all instances of var.

* Sorted eslintrc rules.

* Made eslint rule severity explicit.

* Now prefering const over let.

test/helpers/increaseTime.js

@@ -32,10 +32,10 @@ function increaseTime (duration) {
  * @param target time in seconds
  */
 async function increaseTimeTo (target) {
-  let now = (await latestTime());
+  const now = (await latestTime());
 
   if (target < now) throw Error(`Cannot increase current time(${now}) to a moment in the past(${target})`);
-  let diff = target - now;
+  const diff = target - now;
   return increaseTime(diff);
 }