test: add comprehensive tests for ShellExecutor to ensure command execution and redirection functionality works as expected

This commit introduces a new test suite for the ShellExecutor class, covering various scenarios including command execution, input/output redirection, and error handling. The tests validate the expected behavior of the ShellExecutor, ensuring that commands are executed correctly and that redirection syntax is properly handled. This enhances the reliability of the ShellExecutor and helps prevent regressions in future changes.

Author: tumf