Remove and address sleep statements in `wp-jetpack-plans-spec.js`

[juliaamosova]

Removed 2 sleep statements and addressed it by using waitTillPresentAndDislplayed in respective functions:

• updated clickUpgradeNudge() in wp-admin-jetpack-page.js
• updated clickPurchaseButton() in jetpack-plans-sales-page.js

waitTillPresentAndDislplayed adds the necessary check to make sure that both buttons are displayed and ready to be clicked on.

To test:

The change affects wp-jetpack-plans-spec.js.

[juliaamosova]

The comments tests that are failing in CI are more than likely unrelated but Can create wporg site and connect Jetpack - [PRESSABLE] Gutenberg Markdown block: (desktop) @jetpack Publish a simple post with Markdown block is failing in CI and could be related... Did you test with that host?

Thanks @JavonDavis! The explanation of the failing tests is below:

Failing Jetpack tests:

• 2 are related to the Editor - unrelated to the change I made;
• 2 are related to the Comments - unrelated to the change I made. They seem to be related to https://github.com/Automattic/wp-e2e-tests/pull/1729#issuecomment-449919996

Failing CI test:

• 2 are related to the Comments - same as above - unrelated to the change I made;
• 1 test is affecting wp-admin-jetpack-page.js but not related to the changes I made in thir PR:

Can create wporg site and connect Jetpack - [PRESSABLE] Gutenberg Markdown block: (desktop) @jetpack Publish a simple post with Markdown block:

Timed out waiting for element with css selector of 'a.jp-jetpack-connect__button' to be clickable

The code that doesn't work is here:

https://github.com/Automattic/wp-e2e-tests/blob/b342aa586ac4bb4b939309603c5b5500fb973bd8/lib/pages/wp-admin/wp-admin-jetpack-page.js#L14

There is yet another sleep statement that needs to be addressed. I am already working on it :) It will be addressed in a separate PR since it is a different issue.