[lldb] Remove process restart prompt from TestSourceManager (#85861)

In TestSourceManager, test_artificial_source_location will give the
process restart prompt if you run the test individually. The reason is
that we run the process twice: first using a convenience function to run
to a specific breakpoint and then again to check for a specific message
emitted when you hit the breakpoint. Instead of running twice and making
the test difficult to run individually, we can just check for the
specific messages using other commands.