Work around shell redirection bug on HP-UX /bin/sh.
On HP-UX /bin/sh, when "read" is used in a subshell with redirections,
it seems that some of the input gets dropped on the floor. For example:
% cat >test.txt <<'EOF'
hello
world
EOF
% ( exec 3<test.txt; read a <&3; read b <&3; echo $a $b )
hello
In TEST_CHECK_PAM_MD5, we hit this bug because AT_CHECK runs its
commands in a subshell, and as a result some of the image data is
lost after reading the header and verification fails.
Work around the issue by performing only a single AT_CHECK with a
single redirection.