testConsoleBuf: tighten the check for the /utf-8 flag

Ben Boeckelrequested to merge
ben.boeckel/kwsys:vs2015-utf-8-flag-check into master

It appears that Visual Studio 2015 Update 1 lacks this flag. Update 3 includes it; Update2 is unknown.

See cmake/cmake#16623 (closed).

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