test: Update string check for Xcode 15 compat

[einsteinx2]

Summary

We were previously directly checking the internal obj-c string class name in 2 unit tests. This has changed to the internal swift class name in iOS 17/Xcode 15. I modified the checks to use the more generic isKindOfClass: and now they work on both iOS 17 and earlier versions.

Testing Plan

• [x] Was this tested locally? If not, explain why.

• Tests confirmed using both Xcode 14 and 15

  Reference Issue (For mParticle employees only. Ignore if you are an outside contributor)

• Closes https://go.mparticle.com/work/SQDSDKS-5711

[mparticle-automation]

:tada: This PR is included in version 8.15.0 :tada:

The release is available on GitHub release

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