fix(ui): fixed flakey tests

[asalem1]

Closes #16253

Problem

flush & signin would flake if called multiple times in the same test suite

Solution

consolidated the logic that needed to be retrieved from signing in into the first beforeEach call and removed the multiple usages of cy.flush and cy.signin since the test suite was logging in successfully and was flaking when we would flush the credentials and sign in again due to idiosyncrasies in the way the tests were running

• CHANGELOG.md updated with a link to the PR (not the Issue)

[drdelambre]

so it was already logged in, then broke itself trying to login again?

[asalem1]

so it was already logged in, then broke itself trying to login again?

Something like that. We were also flushing the credentials so if the operation hadn't stopped running, we would all of a sudden lose context and not have specific properties that may have been available at the outset of the test

[drdelambre]

if anything, its simpler and easier to reason about