Fix 'testSignature' browser tests under nightwatch_local

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.

src/app/tabs/runTab/settings.js

@@ -97,7 +97,7 @@ class SettingsUI {
         <div class=${css.account}>
           <select name="txorigin" class="form-control ${css.select}" id="txorigin"></select>
           ${copyToClipboard(() => document.querySelector('#runTabView #txorigin').value)}
-          <i id="remixRunSignMsg" class="fas fa-edit ${css.icon}" aria-hiden="true" onclick=${this.signMessage.bind(this)} title="Sign a message using this account key"></i>
+          <i id="remixRunSignMsg" class="fas fa-edit ${css.icon}" aria-hidden="true" onclick=${this.signMessage.bind(this)} title="Sign a message using this account key"></i>
         </div>
       </div>
     `

test-browser/helpers/contracts.js

@@ -172,7 +172,9 @@ function scrollInto (target) {
 function signMsg (browser, msg, cb) {
   let hash, signature
   browser
+    .waitForElementPresent('i[id="remixRunSignMsg"]')
     .click('i[id="remixRunSignMsg"]')
+    .waitForElementPresent('textarea[id="prompt_text"]')
     .setValue('textarea[id="prompt_text"]', msg, () => {
       browser.modalFooterOKClick().perform(
         (client, done) => {

test-browser/tests/generalTests.js

@@ -34,7 +34,7 @@ function runTests (browser) {
       async.waterfall([function (callback) { callback(null, browser) },
         testSimpleContract,
         testSuccessImport,
-        testFailedImport, /* testGitHubImport */
+        testFailedImport, /* testGitHubImport, */
         addDeployLibTestFile,
         testAutoDeployLib,
         testManualDeployLib,
@@ -168,22 +168,28 @@ function testSignature (browser, callback) {
     contractHelper.signMsg(browser, 'test message', (h, s) => {
       hash = h
       signature = s
+      browser.assert.ok(typeof hash.value === 'string', 'type of hash.value must be String')
+      browser.assert.ok(typeof signature.value === 'string', 'type of signature.value must be String')
       contractHelper.addFile(browser, 'signMassage.sol', sources[6]['browser/signMassage.sol'], () => {
         contractHelper.switchFile(browser, 'browser/signMassage.sol', () => {
           contractHelper.selectContract(browser, 'ECVerify', () => { // deploy lib
             contractHelper.createContract(browser, '', () => {
-              browser.waitForElementPresent('.instance:nth-of-type(4)')
-              .click('.instance:nth-of-type(4) > div > button')
-              .clickFunction('ecrecovery - call', {types: 'bytes32 hash, bytes sig', values: `"${hash.value}","${signature.value}"`}).perform(
+              const instanceSelector = '.instance:nth-of-type(4)'
+              browser.waitForElementPresent(instanceSelector)
+              .click(instanceSelector + ' > div > button')
+              .getAttribute(instanceSelector, 'id', (result) => {
+                // skip 'instance' part of e.g. 'instance0x692a70d2e424a56d2c6c27aa97d1a86395877b3a'
+                const address = result.value.slice('instance'.length)
+                browser.clickFunction('ecrecovery - call', {types: 'bytes32 hash, bytes sig', values: `"${hash.value}","${signature.value}"`}).perform(
                   () => {
                     contractHelper.verifyCallReturnValue(
                       browser,
-                    '0x08970fed061e7747cd9a38d680a601510cb659fb',
+                      address,
                       ['0: address: 0xCA35b7d915458EF540aDe6068dFe2F44E8fa733c'],
                       () => { callback(null, browser) }
                     )
-                }
-              )
+                  })
+              })
             })
           })
         })