Below is the way from the manual:
Use the TestResultSetting object under the Project to configure the path to which Spirent Test Center will write test results.
set trs [lindex [stc::get project1 -children-TestResultSetting] 0]
stc::config $trs -saveResultsRelativeTo NONE -resultsDirectory my_results
Results can be saved relative to one of these four locations:
• INSTALL_DIR - Spirent TestCenter installation directory.
• CURRENT_WORKING_DIR - Current working directory. This is the directory that Spirent TestCenter currently has open. If you change directory (cd) in your script, the working directory will also change.
• USER_WORKING_DIR - User working directory. This is ~ on *NIX and typically %USERPROFILE %/My Documents/Spirent/… on Windows.
• CURRENT_CONFIG_DIR - Current configuration directory. This is the directory where the saved or loaded .xml or .tcc file is located. If no .xml or .tcc file has been saved or loaded, files are saved to the user working directory.
If one of these paths is not suitable, an absolute path can be used.
Note: Absolute paths are not recommended, because the script is no longer portable.
USER_WORKING_DIR is the recommended setting, especially for shared environments.