fix(runner): detect set_up failure when source file does not exist

[Chemaclass]

Summary

Fixes#611

• Fix source of non-existent file in set_up() silently passing all tests
• Save subshell stdout to FD 5 and restore it in EXIT trap to prevent leaked redirect from losing export_subshell_context output
• Add sentinel variable _BASHUNIT_SETUP_COMPLETED to detect unexpected exit during set_up when $? is incorrectly 0

Root cause

When source of a non-existent file fails under set -eE (as used by execute_test_hook):

1. The ERR trap does not fire (Bash behavior for special builtins)
2. set -e kills the subshell, but the { hook } >file 2>&1 redirect from execute_test_hookleaks into the EXIT trap
3. export_subshell_context output goes to the hook output file instead of stdout
4. test_execution_result is empty → no failures detected → test silently passes

[github-actions]

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User:@Chemaclass
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