Port to Solaris 10 /bin/sh.

Problem reported by Dagobert Michelsen in <http://bugs.gnu.org/16380>.
* tests/bre, tests/ere, tests/spencer1-locale:
Prefer re_shell_ to SHELL, if re_shell_ is set.
* tests/init.sh (re_shell_): Export if it's used.
tests: port Solaris 10 /bin/sh patch back to GNU/Linux

Problem reported by Jim Meyering.
* tests/bre, tests/ere, tests/spencer1-locale:
Prefer re_shell, not re_shell_.
* tests/init.sh (re_shell): New var, which is exported instead of
re_shell_.