test: change browser locale to C.UTF-8
test: change browser locale to C.UTF-8

We used to use en_US.UTF-8 as our "always available" English locale, but
it's not always available.  Use C.UTF-8 instead.

This fixes the many warnings from firefox that can be seen when running
unit-tests:

(firefox-esr:16949): Gtk-WARNING **: 14:46:06.423: Locale not supported by C library.
	Using the fallback 'C' locale.

mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit