Updates naming style in test file.

No functional changes in this CL.

* No need for Hungarian Notation, specifically in Kotlin files
* Identifies the instance under test by naming it `underTest`, a common
  best practice in Java unit tests

Bug: b/235403546
Test: N/A
Change-Id: Ib5f38135cd47a748bcba2d4bfada7256570de41a