Fix locale/tst-locale-locpath cross-testing when sshd sets LANG.

The locale/tst-locale-locpath test unsets LANG, then runs a test with
test_wrapper_env and expects LANG to remain unset for that test.  This
does not work for cross-testing with cross-test-ssh.sh when sshd (on
the system specified as an argument to cross-test-ssh.sh) is
configured to have a default LANG setting.

The general design used in cross testing, after commit
8540f6d2 ("Don't require test wrappers
to preserve environment variables, use more consistent environment.",
6 June 2014), is that environment settings required by tests should be
passed explicitly to $(test-wrapper-env).  This patch changes
tst-locale-locpath.sh to pass an explicit LANG= rather than expecting
"unset LANG" to be in effect for the program run under
test_wrapper_env.  Note that this does slightly change the environment
in which the test is run natively (empty LANG instead of unset LANG)
but that difference does not appear relevant to what it is trying to
test.

Tested for Arm that this fixes the failure seen for that test in
cross-testing.