[lldb] Add test to document alias tab completion behaviour (#65760)

While looking at https://github.com/llvm/llvm-project/issues/49528 I
found that, happily, aliases can now be tab completed.

However, if there is a built-in match that will always be taken. Which
is a bit surprising, though logical if we don't want people really
messing up their commands I guess.

Meaning "b" tab completes to our built-in breakpoint alias, before it
looks at any of the aliases. "bf" doesn't match "b", so it looks through
the aliases.

I didn't find any tests for this in the obvious places, so this adds
some.