tests: work even if non-GNU gzip is installed in /usr/local/bin

tests: work even if non-GNU gzip is installed in /usr/local/bin

Problem reported by Antonio Diaz Diaz in
<http://lists.gnu.org/archive/html/bug-gzip/2013-04/msg00004.html>.
* tests/help-version (zdiff_setup, zcat_setup, znew_setup, zgrep_setup)
(gzexe_setup): Pass --__bindir so that subsidiary programs are our
own's, not /usr/local/bin's.  This requires using 'eval' on the result.
(zcmp_setup, gunzip_setup, zmore_setup, zless_setup, zforce_setup)
(zegrep_setup, zfgrep_setup): Invoke one of the other setup functions,
to make the patterns more obvious and simplify future maintenance.
* zcmp.in, zegrep.in, zfgrep.in: Pass __bindir to subsidiary program.