imgtool: fixed keys/general.py to pass existing unittests

keys.KeyClass._emit is able to use 'file' parameter not as a file
but some object (not only sys.stdout but io.StringIO, like by
tests).

Fixed all explicit checks for sys.stdio usage in favor of
io.TextIOBase, also improve a single unit test to cover
also all the changed methods.

Signed-off-by: Denis Mingulov <denis@mingulov.com>