[lldb/test] Fix TestSaveCrashlog.py following changes in eef5eadb

This patch fixes TestSaveCrashlog.py failure introduces by eef5eadb

,
which restricts the number of positional argument for the output file to 1.

I expected to get the output file but `argparse` puts the object in a
list (even by constrained to a singled positional argument).

Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>