gettext: Force UTF-8 runtime and skip requiring ISO-8859-1

gettext: Force UTF-8 runtime and skip requiring ISO-8859-1

On musl, there is no real legacy (non-UTF-8) fr_FR locale.
These tests are designed for libcs that ship both fr_FR (ISO-8859-1)
and fr_FR.UTF-8. So the right thing will be to SKIP these tests

Unsetting LOCALE_FR will ensure that it does not enable ISO-8859-1
path, so reset it in run-ptest but thats not enough because it is
being set in the test's own init-env file as well so clear it in that
file as well.

Fixes
Failed ptests:
{'gettext': ['intl-2', 'intl-4', 'intl-thread-3', 'lang-sh', 'lang-bash']}

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>