Refactor UI-E2E tests

[rzadp]

https://github.com/poanetwork/tokenbridge/pull/186#discussion_r313578915

The UI-E2E tests are unclear because of two reasons:

1. They rely on sequential execution - for example first test case opens the website, and subsequent test cases assume that the website is opened.
2. The pre-conditions like selected MetaMask network are not clearly specified - it is not clear which test cases should begin with foreign network, and which with home network.

Expected changes

• Wrap test cases that rely on sequential execution into a test.describe with test.before
• Current sequential execution:
  • Opening website URL and relying on open website in subsequent tests
  • Performing the transfer and checking the balances in subsequent tests
  • Switching MetaMask networks and relying on it in subsequent tests