[lldb] Update test.rst with a paragraph about pdb

Debugging tests sometimes involves debugging the Python source. This adds a paragraph to
the "Debugging Test Failures" section about using `pdb`, and also describes how to run
lldb commands from pdb.

Differential Revision: https://reviews.llvm.org/D99744

GitOrigin-RevId: 0c653d4c3d1426267337576ab202bb594144111c
1 file changed