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

[kastiglione]

Summary

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.

[JDevlieghere]

Thanks, this is super useful. LGTM modulo the inline comment.

lldb/docs/resources/test.rst

387

This should be indented to show up as code.

[kastiglione]

Indent source block

lldb/docs/resources/test.rst

387

thanks for catching it

[kastiglione]

s/lldb alias/pdb alias

lldb/docs/resources/test.rst

 page describes how to setup an emulation environment using QEMU helper scripts
 found under llvm-project/lldb/scripts/lldb-test-qemu. These scripts currently
 work with Arm or AArch64, but support for other architectures can be added easily.
 
 Debugging Test Failures
 -----------------------
 
 On non-Windows platforms, you can use the ``-d`` option to ``dotest.py`` which
-will cause the script to wait for a while until a debugger is attached.
+will cause the script to print out the pid of the test and wait for a while
+until a debugger is attached. Then run ``lldb -p <pid>`` to attach.
+
+To instead debug a test's python source, edit the test and insert
+``import pdb; pdb.set_trace()`` at the point you want to start debugging. In
+addition to pdb's debugging facilities, lldb commands can be executed with the
+help of a pdb alias. For example ``lldb bt`` and ``lldb v some_var``. Add this
+line to your ``~/.pdbrc``:
+
+::
+
+   alias lldb self.dbg.HandleCommand("%*")
+
+::
 
 Debugging Test Failures on Windows
 ``````````````````````````````````
 
 On Windows, it is strongly recommended to use Python Tools for Visual Studio
 for debugging test failures. It can seamlessly step between native and managed
 code, which is very helpful when you need to step through the test itself, and
 then into the LLDB code that backs the operations the test is performing.