gen-strtab.awk: Work around HP-UX shell bug in test case.

On HP-UX /bin/sh, redirections on "read" in a subshell can drop some
input on the floor.  Since Autotest runs many things in subshells, we
hit this when trying to construct the program to dump the string table.

Rearranging the code to use only a single redirection works around
the problem.