tests: restrict shells to those that support 'local'
The local keyword is very widely supported and used
in tests in coreutils and grep at least. Therefore
restrict to testing with shells that support it.
This mainly excludes /bin/sh on Solaris.
Note standard ksh also doesn't support this keyword,
but that wasn't in the list of considered shells anyway.
For discussion on adding 'local' to POSIX, see:
http://thread.gmane.org/gmane.linux.debian.devel.bugs.general/122267/focus=3271
* tests/init.sh (gl_shell_test_script_): Add a test for 'local'.