The `testSignature()` browser test in `generalTests.js` would
occasionally fail due to not obtaining the correct `hash` and
`signature` values from the browser, resulting in:
```
✖ generalTests
   - Simple Contract (40.954s)
   Failed [equal]: ('' == '0: address: 0xCA35b7d915458EF540aDe6068dFe2F44E8fa733c')  - expected "0: address: 0xCA35b7d915458EF540aDe6068dFe2F44E8fa733c" but got: ""
       at Object.<anonymous> (/home/scottt/work/remix-0.8/remix-ide/test-browser/helpers/contracts.js:129:22)
```
failures.

This patch fixes it by waiting for the DOM elements with
`waitForElementPresent` which implicitly retries and obtaining
the correct contract instance address instead of hard coding it.