Fix getElementText util function

[mmarkelov]

close #21
I believe that we got only this cases:

• await expect(page).toHaveText('test') 2 arguments, look through body to contain 'test'
• await expect(page).toHaveText('.selector', 'test') 3 arguments, wait for .selector, look through .selector to contain 'test'
• await expect(page).toHaveText('.selector', 'test', options) 4 arguments, wait for .selector with passed options, look through .selector to contain 'test'
• await expect(element).toHaveText('test') 2 arguments, look through element to contain 'test'

[mmarkelov]

@mxschmitt what do you think about possible cases? I'll fix tests and add some more for this cases tomorrow.
I just think that there is no much sense in this case:

await expect(element).toHaveText('test', options)

cause I just don't have any ideas where should we pass them

[mxschmitt]

lets see how it goes!