ci: generate multibyte locales for the GNU testsuite

[sylvestre]

Without en_US.UTF-8, ru_RU.UTF-8 and el_GR.iso88597 the testsuite silently skips eight multibyte tests, hiding both regressions and improvements. Also make require_el_iso88597_locale_ probe locale -a instead of always skipping.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.01%. Comparing base (b038f8f) to head (ff2f162).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #539   +/-   ##
=======================================
  Coverage   83.01%   83.01%           
=======================================
  Files          13       13           
  Lines        7037     7037           
  Branches      401      401           
=======================================
  Hits         5842     5842           
  Misses       1192     1192           
  Partials        3        3           

Flag	Coverage Δ
macos_latest	83.65% <ø> (ø)
ubuntu_latest	83.86% <ø> (ø)
windows_latest	0.00% <ø> (ø)

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[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 65 / PASSED: 14 / FAILED: 43 / SKIPPED: 8
  Reference: TOTAL: 65 / PASSED: 14 / FAILED: 41 / SKIPPED: 10

Changes from main branch:
  TOTAL: +0
  PASSED: +0
  FAILED: +2

New test failures (2):
  - title-case
  - utf8-ru