tests: improve test coverage for ls stat checks

tests: improve test coverage for ls stat checks

* tests/ls/stat-free-color.sh: Check for the availability
of various stat calls individually, and add statx() and fstatat64()
to the list to check.  Fix the stat counting logic to
ignore lines like "+++ exited with 0 +++".
* tests/ls/stat-free-symlinks.sh: Check syscalls other than stat().