[llvm-symbolizer] improve test and fix doc example after recent...

[llvm-symbolizer] improve test and fix doc example after recent --print-source-context-lines behaviour change

I believe that after https://reviews.llvm.org/D102355 the behaviour of --print-source-context-lines has changed.

Before: --print-source-context-lines=3 prints 4 lines.
After: --print-source-context-lines=3 prints 3 lines.

Adjust the example in the docs for this change and make the testing a little more robust.

Differential Revision: https://reviews.llvm.org/D104114