Testing: Re-enable and refactor login tests that don't use magic links

[rachelmcr]

In #11283 we temporarily disabled all login and signup connected tests.

This PR re-enables the login tests that don't use magic links, since they should not have any issues on their own. It also refactors these tests to make the test steps clearer for each test.

Background: These tests were sometimes failing because the magic link tests rely on an activity test rule to be able to invoke the magic link intent directly, and that intent can't always be launched. (The cause for that is still under investigation.) Because the test rule for that intent is launched before every test in the LoginTests class, it can sometimes cause unrelated tests to fail. This PR keeps that test rule and the magic link login test disabled, so it shouldn't cause any issues for the re-enabled tests.

As we work on the magic link issue, we could consider putting all tests that use magic links into their own class (rather than separating tests based on login and signup exclusively) so the test rule is only applied to tests that need it.

To test:

• Run the optional connected tests on CI and confirm they pass.
• You can also run the connected tests locally with ./gradlew :WordPress:connectedVanillaDebugAndroidTest. (Note that local test runs include the WPScreenshotTest, which isn't included in the CI run. That test is currently failing, but it's also failing in develop and the failure is related to mocks, not login. We can fix that it a separate PR.)

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

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[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[oguzkocer]

Thanks for working on this @rachelmcr!