Perhaps I was impatient; on Hydra, the test suite does fail after >1
hour:

[...]
PASS: file_filter_test
PASS: grub_cmd_tr
FAIL: grub_func_test
============================================================================
Testsuite summary for GRUB 2.02
============================================================================
# TOTAL: 80
# PASS:  56
# SKIP:  23
# XFAIL: 0
# FAIL:  1
# XPASS: 0
# ERROR: 0
============================================================================
See ./test-suite.log
Please report to bug-grub@gnu.org
============================================================================
make[4]: *** [Makefile:11019: test-suite.log] Error 1
make[4]: Leaving directory '/tmp/guix-build-grub-2.02.drv-0/grub-2.02'
make[3]: *** [Makefile:11127: check-TESTS] Error 2
make[3]: Leaving directory '/tmp/guix-build-grub-2.02.drv-0/grub-2.02'
make[2]: *** [Makefile:11916: check-am] Error 2
make[2]: Leaving directory '/tmp/guix-build-grub-2.02.drv-0/grub-2.02'
make[1]: *** [Makefile:10904: check-recursive] Error 1
make[1]: Leaving directory '/tmp/guix-build-grub-2.02.drv-0/grub-2.02'
make: *** [Makefile:11918: check] Error 2
phase `check' failed after 3637.0 seconds
[...]

https://hydra.gnu.org/build/2414840