fix(frame): Fix Frame.waitFor's XPath pattern detection

[xprudhomme]

Up to now, only strings starting with '//' are considered as to XPath selectors.

Unfortunately, this is too restricting and leads to errors especially when asking the page to wait for XPath selectors not starting with '//'.

If the XPath is something like '(//div)[4]/@itemprop' then it is passed to the waitForSelector method and eventually document.querySelector, which in turn throws an error because it does not recognize it as a valid CSS selector.

Because of this, I had to implement something like this in my code:

await isXPathSelector(selector)
            ? page.waitForXPath(selector)
            : page.waitForSelector(selector);

This fix allows the use of valid XPath selectors starting with parenthesis and to be recognized as such by the Frame's waitFor method.

[mathiasbynens]

This needs tests.

[xprudhomme]

It's my first PR here. Never done any test for this before.
Any help appreciated for implementing required tests.
Thanks :)

[mathiasbynens]

Look for the existing waitForXPath tests and add some examples that failed without your patch, but now work.

[xprudhomme]

Ok thanks.

I've changed the xPathPattern to allow for the use of XPath starting with not only "//" but also with an opening parenthesis.

In my last commit, I've also added a new test accordingly.

[jschfflr]

@xprudhomme Thanks for your contribution! Please resolve the merge conflicts and I'm happy to get this landed!

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[jschfflr]

@googlebot I consent.