Add GDBServer Integration Tests (CI)

[PhilippvK]

In preparation to merging #161, I wanted to add some gdbserver-related smoke tests to our ci.

Before this can be merged a few TODOs need to be resolved:

• Support building examples using Clang/LLVM
• Support debugging with lldb
• Improve script/gdbserver_test.cmd and regular expressions.

In the future we might want to add tests for other Plugins such as PrintInstruction or dbusAccess.csv.

[thomasgoodfellow]

Apologies for tardy feedback; wanted toi run this against a working seal5 LLDB and with current pyenv config getting a properly patched build without a broken libpython dependency stalled the task (finally bypassed with LLDB_ENABLE_PYTHON: false in llvm.yml). Here's the simplest set of LLDB equivalents - note that the GDB command aliases are only for the short forms, eg "b" works but not "break". These aliases are built-in and I imagine stable, but the native equivalents for "bt" and "b" are "thread backtrace" and "breakpoint set -name" (rational, orthogonal, but too verbose)

gdb-remote 2001
bt
register read
b main
b _exit
continue
bt
register read
continue
bt
register read
register read x0
register read zero
register read pc