feat(e2e): add reliable input clearing function

refs #616

e2e/calculator.po.ts

-import { by, element, ElementFinder, browser, protractor, ElementArrayFinder } from "protractor";
+import { by, element, ElementFinder, browser, protractor, ElementArrayFinder, Key } from "protractor";
 import { scrollPageToTop, scrollToElement } from "./util.po";
 
 export class CalculatorPage {
@@ -458,4 +458,15 @@ export class CalculatorPage {
     getCalculatorHelpButton() {
         return element(by.css("#help-calc"));
     }
+
+    /**
+     * reliable input clearing
+     */
+    async clearInput(inp: ElementFinder) {
+        const txt = await inp.getAttribute('value');
+        const len = txt.length;
+        for (let n = 0; n < len; n++) {
+            await inp.sendKeys(Key.BACK_SPACE);
+        }
+    }
 }