Add activity manager tests

[elsa-bismuth]

Add activity tests

♻️ Current situation & Problem

• no tests for activity manager
• cannot delete and edit an activity (was removed in PR 122)

⚙️ Release Notes

• added tests for activity manager
• removed swipe between tabs for activity and added swipe for edit and delete activity

📚 Documentation

Please ensure that you properly document any additions in conformance to Spezi Documentation Guide.
You can use this section to describe your solution, but we encourage contributors to document your reasoning and changes using in-line documentation.

✅ Testing

Tested activity manager

📝 Code of Conduct & Contributing Guidelines

By submitting creating this pull request, you agree to follow our Code of Conduct and Contributing Guidelines:

• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 62 lines in your changes missing coverage. Please review.

Project coverage is 35.20%. Comparing base (fca28ca) to head (6c8a960).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Stanford360/Activity/View/ActivityCardView.swift	0.00%	37 Missing ⚠️
Stanford360/Activity/View/ActivityAddView.swift	0.00%	13 Missing ⚠️
Stanford360/Activity/View/ActivityTabView.swift	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   34.99%   35.20%   +0.21%     
==========================================
  Files         104      104              
  Lines        4579     4620      +41     
==========================================
+ Hits         1602     1626      +24     
- Misses       2977     2994      +17     

Files with missing lines	Coverage Δ
Stanford360/Activity/View/ActivityTabView.swift	0.00% <0.00%> (ø)
Stanford360/Activity/View/ActivityAddView.swift	0.00% <0.00%> (ø)
Stanford360/Activity/View/ActivityCardView.swift	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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