fix filepath handling in Get command

[jacalata]

@W-13757625 - the command threw an error and failed if the filepath to save to contained a full stop, e.g
c:\users\chevy.chase\

Fixed by using os.path instead of manual parsing. Also added a test to cover the scenario.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	7	7	7	0%
tabcmd.py	14	14	14	0%
version.py	6	3	3	50%
tabcmd/commands
commands.py	10	10	10	0%
constants.py	72	13	13	82%
server.py	126	26	26	79%
tabcmd/commands/auth
session.py	360	42	42	88%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	117	13	13	89%
delete_command.py	60	16	16	73%
export_command.py	128	28	28	78%
get_url_command.py	189	61	61	68%
publish_command.py	83	25	25	70%
runschedule_command.py	21	7	7	67%
tabcmd/commands/extracts
create_extracts_command.py	42	8	8	81%
decrypt_extracts_command.py	27	2	2	93%
delete_extracts_command.py	37	6	6	84%
encrypt_extracts_command.py	27	2	2	93%
extracts.py	20	2	2	90%
reencrypt_extracts_command.py	27	2	2	93%
refresh_extracts_command.py	48	13	13	73%
tabcmd/commands/group
create_group_command.py	29	5	5	83%
delete_group_command.py	27	2	2	93%
tabcmd/commands/project
create_project_command.py	46	8	8	83%
delete_project_command.py	35	4	4	89%
publish_samples_command.py	28	4	4	86%
tabcmd/commands/site
create_site_command.py	34	5	5	85%
delete_site_command.py	28	2	2	93%
edit_site_command.py	38	2	2	95%
list_command.py	44	10	10	77%
list_sites_command.py	30	2	2	93%
tabcmd/commands/user
create_site_users.py	57	10	10	82%
create_users_command.py	60	10	10	83%
delete_site_users_command.py	43	5	5	88%
user_data.py	220	30	30	86%
tabcmd/execution
_version.py	2	2	2	0%
global_options.py	144	24	24	83%
localize.py	67	8	8	88%
logger_config.py	42	6	6	86%
tabcmd_controller.py	35	4	4	89%
TOTAL	2607	443	83%

[bcantoni]

Overall looks good, just that one question.

Also the mypy step is throwing this error:
tests/e2e/online_tests.py:314: error: Name "test_list_workbooks" already defined on line 308 [no-redef]

I think that was from a previous change, but we should fix it. (Looks like a copy & paste error.)