encoding: Update handling of encoding in tests

Always run tests with encoding=utf-8, regardless of user locale
Don't set &encoding after startup in tests

Helped-By: Michael Reed <m.reed@mykolab.com>

test/functional/legacy/utf8_spec.lua

@@ -10,7 +10,6 @@ describe('utf8', function()
   it('is working', function()
     insert('start:')
 
-    execute('set encoding=utf-8')
     execute('new')
     execute('call setline(1, ["aaa", "あああ", "bbb"])')