My Site Dashboard: Dashboard data source unit tests

[hassaanelgarem]

Part of #18308

Description

This adds and fixes tests related to #18309 and #18310

• Fixes tests build errors in BlogDashboardServiceTests
• Adds DashboardCardTests

Testing Instructions

N/A

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below
• Then installing the build number pr18311-9cf84d1 from App Center on your iPhone

The .ipa file can also be downloaded directly here.
If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below
• Then installing the build number pr18311-9cf84d1 from App Center on your iPhone

The .ipa file can also be downloaded directly here.
If you need access to App Center, please ask a maintainer to add you.