For #6980: Update Private browsing card.

[mcarare]

Added test to description.
Added link to settings.
Updated unit tests.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-io]

Codecov Report

Merging #8348 into master will decrease coverage by <.01%.
The diff coverage is 7.14%.

@@             Coverage Diff              @@
##             master    #8348      +/-   ##
============================================
- Coverage      18.6%   18.59%   -0.01%     
  Complexity      452      452              
============================================
  Files           316      316              
  Lines         12435    12443       +8     
  Branches       1634     1634              
============================================
+ Hits           2313     2314       +1     
- Misses         9923     9930       +7     
  Partials        199      199

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/org/mozilla/fenix/home/HomeFragment.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...ix/home/sessioncontrol/SessionControlInteractor.kt	81.81% <0%> (-3.9%)	18 <0> (ø)
...fenix/home/sessioncontrol/SessionControlAdapter.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
.../onboarding/OnboardingPrivateBrowsingViewHolder.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...la/fenix/home/sessioncontrol/SessionControlView.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...ix/home/sessioncontrol/SessionControlController.kt	66.08% <100%> (+0.29%)	0 <0> (ø)	⬇️

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