[lldb/test] Fix TestSaveCrashlog.py following changes in eef5eadbe6

This patch fixes TestSaveCrashlog.py failure introduces by eef5eadbe6,
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>

lldb/examples/python/crashlog.py

@@ -1274,7 +1274,7 @@ def save_crashlog(debugger, command, exe_ctx, result, dict):
         return
     target = exe_ctx.target
     if target:
-        out_file = options.output
+        out_file = options.output[0]
         identifier = target.executable.basename
         process = exe_ctx.process
         if process: