fixed TaskScriptRunner which may locate the wrong script

[yhwen]

Fixes # .

Description

Fixed several issues with the TaskScriptRunner:

• Instead of using the site_name for searching the script on the site, change to use the custom_dir from the site workspace.
• searching the script under the custom_dir, instead of the os.getcwd(), otherwise it may locate the script from a different job;
• Change the unit tests to enable running the unit tests from different sub-folders, otherwise the unit tests can only be run from the NVFlare root folder.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[chesterxgchen]

LGTM

[chesterxgchen]

/build

[chesterxgchen]

/build