2.3310:  add testcases for JSON::XS RT #84244, double-encoding with utf8

t/01_utf8.t (use utf8), t/14_latin1.t (no utf8)
t/01_utf8.t: enable and fix more 5.6 testcases

Summary: I found no double-encoding problem in JSON::XS/Cpanel::JSON::XS
but clarified more 5.6 cases with mb chars