Fix/apptester

[kozlovsky]

Currently, the CodeExecutor of Tribler used by ApplicationTester does not catch errors properly. To catch errors during the execution of the action, it temporarily replaces stdout and stderr with StringIO objects and then attempts to detect an error by analyzing the output of stderr.

With the current Tribler code, this approach does not work correctly. To write errors to stderr, Python logger creates StreamHandler object that caches reference to stderr. When CodeExecutor replaces stderr to StringIO object, StreamHandler continues to write errors to the original stderr stream, and the errors go unnoticed by CodeExecutor.

This PR fixes error detection in CodeExecutor. Instead of replacing the stderr stream with the StringIO object, it defines the showtraceback hook that catches exceptions arising during the ApplicationTester action.

Also, this PR fixes reporting GUI errors to stderr (sometimes, the error was suppressed by logic in the gui_error handler).

The last change is an automatic screenshot taking when an exception occurs in the CodeExecutor action evaluation. If the application tester passes the app_tester_dir value to the script, it is used as a root directory for screenshot saving.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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