[SDK-2097] Added Prefhelper tests

[nsingh-branch]

Reference

SDK-2097 -- iOS Test coverage

Summary

Added tests to ensure prefhelper values are being properly set and retrieved. Also updated the testbed's Branch Key.

Motivation

Increase the iOS SDK's test coverage.

Type Of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing Instructions

Run the tests and confirm the succeed locally and on GHA.

cc @BranchMetrics/saas-sdk-devs for visibility.

[codecov]

Codecov Report

Merging #1308 (97ca6bb) into master (60b8a62) will increase coverage by 0.09%.
Report is 23 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1308      +/-   ##
==========================================
+ Coverage   50.48%   50.57%   +0.09%     
==========================================
  Files          68       68              
  Lines       10106    10109       +3     
  Branches     3889     3889              
==========================================
+ Hits         5102     5113      +11     
+ Misses       4758     4748      -10     
- Partials      246      248       +2     

see 11 files with indirect coverage changes