…ewer cypress test (elastic#162839)

## Summary

Ref: elastic#162569

The test was trying to load the exceptions tab before the rule details
page loaded. Now we wait for the rule tab to load before continuing.
Something I was unaware of was that `cy.url()` will [automatically
retry](https://docs.cypress.io/api/commands/url#Assertions) until all
chained assertions have passed, which I think can be an easy way to fix
future flake issues where cypress tries to click on an element before
the new page loads.

(cherry picked from commit 7b3cc8f)

# Conflicts:
#	x-pack/plugins/security_solution/cypress/e2e/exceptions/rule_details_flow/read_only_view.cy.ts

…-ref HEAD~1..HEAD --fix'