tests/sys/posixshm/posixshm_test.c::accounting fix after st_size changes

[kib]

Summary

st_blksize is defined by POSIX as the 'preferred I/O block size for this
object.' It is wrong to use st_blksize as the unit for st_blocks and
expect it to be equal to the object size regardless of the change of
st_blksize.

Fixes:  3a1bf59d195c ("shmfd: consistently return size in 512 byte blocks for fstat(2) st_blocks")

[kib]

Commit all chunks

[kib]

Cosmetics: use ATF_CHECK_EQ instead of ATF_REQUIRE for size comparison.
Add debugging printfs to make debugging tests not so painful.