[lldb][NFC] Refactor TestSafeFuncCalls.py

Summary:
This patch inlines all the single-line functions that we only use once in the test
and replaces the assertTrue with an assertEquals to improve the error message
when this test fails.

Reviewers: JDevlieghere

Reviewed By: JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D75497