tslint: Enable no-var-requires

Rationale from the rule's page:

AMD-style require([]) and CommonJS-style require("") statements are
environment-specific and more difficult to statically analyze.

ES2015-style imports are part of the JavaScript language specfication
and recommended as the path going forward. TypeScript will compile them
to environment-specific forms as needed.

test/index.ts

@@ -10,7 +10,7 @@
 // to report the results back to the caller. When the tests are finished, return
 // a possible error to the callback or null if none.
 
-const testRunner = require('vscode/lib/testrunner');
+import * as testRunner from "vscode/lib/testrunner";
 
 // You can directly control Mocha options by uncommenting the following lines
 // See https://github.com/mochajs/mocha/wiki/Using-mocha-programmatically#set-options for more info