test: convert var->const/let in tests

Overview

Use eslint to convert all var to const/let in test/, manually fix anything that eslint messed up.

If we're going to go ES6 in test/, we might as well go all the way.

To fix rules with eslint

Apply this:

diff --git a/.eslintrc b/.eslintrc
index cf1f36c86b..e09fd6ac64 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -110,6 +110,8 @@ rules:
 
   # ECMAScript 6
   # http://eslint.org/docs/rules/#ecmascript-6
+  no-var: error
+  prefer-const: error
   arrow-parens: [2, always]
   arrow-spacing: [2, {before: true, after: true}]
   constructor-super: 2
@@ -119,7 +121,6 @@ rules:
   no-dupe-class-members: 2
   no-new-symbol: 2
   no-this-before-super: 2
-  prefer-const: [2, {ignoreReadBeforeAssign: true}]
   rest-spread-spacing: 2
   template-curly-spacing: 2

Run this:

eslint --fix --rulesdir=tools/eslint-rules "test/**/*.js"

---

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test