Add test to verify customize firefox tab displays

[krishpdev]

Relevant Links

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1976542
TestRail: 134463

Description of Code / Doc Changes

Leave a bullet-pointed list of changes you made.

Verifies that the customize toolbar tab is correctly displayed. Verifies it is correctly open in the background as well as in focus.

Process Changes Required

Mark the relevant boxes:

• Adds a dependency (rerun pipenv install)
• Changes the BasePage
• Changes or creates a BOM/POM (name the object model): _
• Changes CI flow
• Changes scheduled Beta or DevEdition
• Changes Git hooks or Github settings
• Changes L10n harness

Screenshots or Explanations

If you need to explain your code, do it here.

Comments or Future Work

Do we need to start another PR soon to address something you saw while working on this?

Workflow Checklist

• Please request reviewers
• If this is an unblocker, please post in Slack.
• If asked to address comments, please resolve conversations.
• If asked to change code, please re-request review from the person who wanted changes.

Thank you!

[Tracy-Walker]

Krish, Looks good with one exception. The element in line 46.. Please make that a selector as part of modules/data/customize_firefox.components.json, like
"customize-page": {
"selectorData": "customization-container",
"strategy": "id",
"groups": []
},
Forgive white spacing badness. Make it look like the others in that file.
Then line 46 becomes: element = tabs.get_element("customize-page").

[Tracy-Walker]

looks good