Show warning on generation when leave out `.url` attribute for amOnPage

[okolesnyk]

General summary

We noticed on our team after 2.1.0 release that amOnPage action previously assumed the .url attribute from the page, but then after needed to be explicitly included (all our tests failed).

Steps to reproduce

1. Create test with amOnPage action:
   <amOnPage url="{{AdminLoginPage}}" stepKey="goToAdminLoginPage"/>
2. Generate tests by running: vendor/bin/robo generate:tests

Expected result

1. Warning message appears: Can not resolve argument on url attribute for amOnPage action. Step Key: "goToAdminLoginPage".

Actual result

1. Tests successfully generated and action amOnPage looks like this:
   $I->amOnPage("{{AdminLoginPage}}");

[tomreece]

I was able to reproduce this issue. @okolesnyk I understand we have some of these issues around for community members to tackle, but I don't like the idea of these issues sitting for so long (over a year in this case).

I would like to close this issue and create a Jira ticket instead. Are you ok with that?

[tomreece]

Internal ticket
https://jira.corp.magento.com/browse/MQE-1799

[lbajsarowicz]

Fixed. @tomreece & @okolesnyk - Please verify #546 PR.