These inexplicably avoid segfaults with gcc-12.2.0.
* lib/tests/posix/50-open-read.c (main)[__GNUC__ > 11]: Use malloc
instead of local allocated string.
* lib/tests/scaffold/7i-struct-struct.c (main)[__GNUC__ > 11]:
Initialize all fields of struct.
* lib/tests/scaffold/t.c (main): Likewise.
* build-aux/test-c.sh: Support running by hand.
* lib/tests/posix/50-open-read.c: New test.
* lib/tests/posix/data/open-read: New file.
* build-aux/check-mescc.sh (TESTS): Add it.