tests: fix "comparison between signed and unsigned" compilation warnings
preadv-pwritev.c: In function 'main':
preadv-pwritev.c:162: warning: comparison between signed and unsigned
readv.c: In function 'main':
readv.c:148: warning: comparison between signed and unsigned
recvmsg.c: In function 'main':
recvmsg.c:148: warning: comparison between signed and unsigned