tests: `waitForPromise()` test improvements

* Avoid a false positive if a Promise that is expected to reject
    doesn't reject.
  * Use modern JavaScript language features: arrow functions,
    `const`/`let` instead of `var`.
  * Remove the tests that test Promise behavior.
  * Add new test that checks that it returns a Promise.

tests/frontend/specs/helper.js

@@ -216,49 +216,30 @@ describe("the test helper", function(){
   });
 
   describe('the waitForPromise method', function() {
+    it('returns a Promise', async function() {
+      expect(helper.waitForPromise(() => true)).to.be.a(Promise);
+    });
+
     it('takes a timeout and waits long enough', async function() {
       this.timeout(2000);
-      var startTime = Date.now();
-      await helper.waitForPromise(function() {
-        return false;
-      }, 1500).catch(function() {
-        var duration = Date.now() - startTime;
-        expect(duration).to.be.greaterThan(1490);
-      });
+      const startTime = Date.now();
+      let rejected;
+      await helper.waitForPromise(() => false, 1500)
+          .catch(() => { rejected = true; });
+      expect(rejected).to.be(true);
+      const duration = Date.now() - startTime;
+      expect(duration).to.be.greaterThan(1490);
     });
 
     it('takes an interval and checks on every interval', async function() {
       this.timeout(4000);
-      var checks = 0;
-      await helper.waitForPromise(function() {
-        checks++;
-        return false;
-      }, 2000, 100).catch(function() {
-        expect(checks).to.be.greaterThan(15);
-        expect(checks).to.be.lessThan(21);
-      });
-    });
-
-    describe('returns a Promise', function() {
-      it('calls then after success', async function() {
-        let called = false;
-        await helper.waitForPromise(function() {
-          return true;
-        }).then(function() {
-          called = true;
-        });
-        expect(called).to.be(true);
-      });
-
-      it('calls catch on failure', async function() {
-        let called = false;
-        await helper.waitForPromise(function() {
-          return false;
-        },0).catch(function() {
-          called = true;
-        });
-        expect(called).to.be(true);
-      });
+      let checks = 0;
+      let rejected;
+      await helper.waitForPromise(() => { checks++; return false; }, 2000, 100)
+          .catch(() => { rejected = true; });
+      expect(rejected).to.be(true);
+      expect(checks).to.be.greaterThan(15);
+      expect(checks).to.be.lessThan(21);
     });
   });