[libcxx][pretty printers] Check GDB Python scripting support

[libcxx][pretty printers] Check GDB Python scripting support

I found this after upgrading from Ubuntu bionic (gdb 8.1.1) to
Focal (gdb 9.2). (where this test fails, but that's for a
different patch)

9.2 allows you to set breakpoint commands from
Python, which was added in 8.3.
(bintutils a913fffbdee21fdd50e8de0596358be425775678
"Allow breakpoint commands to be set from Python")

The reason this test never failed before was because it did so
silently. "source <python file>" doesn't fail even if that script
raises an Exception.

To fix this extend the gdb lit feature to check that:
* gdb exists
* has Python support
* allows you to set breakpoint commands

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D110334