test: fix shfmt error message assertions

[kolyshkin]

The formatter tests fail against shfmt >= 3.13 (I hit this on Fedora with shfmt 3.13.1, on current main, unrelated to any local change).

Changed error messages

mvdan/sh 16cc925dc ("syntax: consistently quote tokens in error messages") landed in shfmt 3.13.0, so two messages the tests match on now look different:

-  parse-problems.sh:10:1: > must be followed by a word
+  parse-problems.sh:10:1: `>` must be followed by a word

-  shfmt.sh:25:14: the "function" builtin is a bash feature; tried parsing as posix
+  shfmt.sh:25:14: the `function` builtin is a bash feature; tried parsing as posix

The assertions now accept either quoting style, alongside the pre-existing alternative for much older shfmt versions.

CI does not see this because verify.yml installs shfmt via apt-get, and the version in the Ubuntu archive predates 3.13.

Missing await

While tracking this down: the three expect(...).rejects.toThrow(...) assertions in this file were never awaited or returned. The test therefore finished green and the rejection surfaced during whichever test ran next — so jest blamed should format when shfmt is present for a message belonging to should throw when parsing using the wrong language dialect, which made this a good deal more confusing than it needed to be.

Verified the assertions actually bite now: deliberately corrupting the expected strings fails the tests that own them, rather than their neighbours.

[kolyshkin]

Opened #1394 which is this PR plus the CI addition to also test against the latest shfmt.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.56%. Comparing base (e46e4a6) to head (d14b5d2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1393   +/-   ##
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  Coverage   80.56%   80.56%           
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  Files          29       29           
  Lines        1513     1513           
  Branches      374      374           
=======================================
  Hits         1219     1219           
  Misses        235      235           
  Partials       59       59           

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