[ThreadPlan] Add a test for `thread step-in -r`, NFC

Adds test coverage for ThreadPlanStepInRange::SetAvoidRegexp.

See:
http://lab.llvm.org:8080/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/lldb/source/Target/ThreadPlanStepInRange.cpp.html#L309

Differential Revision: https://reviews.llvm.org/D91220