fix(selftests): stop the pipefail selftest heading from dumping the shell environment

[LukasWodka]

One heading in scripts/tests/pipefail-early-close-selftest.sh (line 160) was written in double quotes with backticks around set, so bash ran the builtin set as a command substitution and the heading printed the entire shell environment — every variable, including any tokens present in the runner environment, and every function body — instead of the words. Assertions were never affected; only the log was.

Fix: single-quote the heading so it prints literally. The only other backtick heading in the selftests (line 771) was already escaped; grep -nE '(echo|printf) +"[^"]*\' scripts/tests/*.sh` finds nothing else.

Verification: bash -c on the new line prints the heading verbatim; shellcheck -S warning -x and bash -n clean; make selftest-pipefail-early-close locally: 168 passed, 0 failed — the heading now appears as text with nothing following it.

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Note

Low Risk
Test-only logging fix with no production or assertion behavior changes.

Overview
Fixes a selftest log heading that accidentally ran the shell set builtin via backticks inside double quotes, so CI/local logs printed the entire environment instead of the section title.

The change single-quotes the echo for == a \set` line carries code too ...` so backticks are literal. Test assertions are unchanged; only output formatting is fixed.

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[LukasWodka]

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