scripts: harden sftp.test

[ejohnstown]

The ready-file counter is shared by every create_port call and was never reset, so the twenty iterations are a budget for the whole run rather than per server start. A slow first start leaves later ones with no wait at all, and the test fails with "NO ready file" instead of waiting. The guard after the loop then disagreed with the loop as well: the wait is for a non-empty file, but the guard only asked whether the file existed, so a wait that ran out still went on to read an empty port.

• reset the counter in create_port, so each server start gets the full wait and a new call site cannot forget it
• test the guard on -s, matching what the loop waited for
• drop echo -e, undefined for POSIX sh and printed literally by shells that do not take the flag; the escapes were only ever blank lines
• quote the expansions, use $(...) instead of backticks, and grep -q
• exit 1 rather than exit -1 from the trap handler
• drop the redundant PWD assignment before the set-directory test, and the second copy of the wolfsftp executable check

Leaves shellcheck -s sh clean apart from SC2329 on the trap handler.

[Copilot]

Pull request overview

This PR hardens the scripts/sftp.test POSIX sh test runner to reduce flakiness around server startup/ready-file handling and to improve portability and robustness of the shell code.

Changes:

• Reset the ready-file wait counter per create_port invocation and align the post-loop guard with the loop’s -s condition.
• Remove non-POSIX echo -e usage and apply safer shell practices (quoting, $(...), grep -q, exit 1 in trap).
• Simplify redundant checks/assignments in the test flow.

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[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #1206

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