test: Explicitly set encoding to utf8 when opening text files

These are text files but their encoding does not depend on the locale.
Not all of them require utf8 but it is better to fix it at something
to remove potential unpredictability.

This is necessary on FreeBSD where no locale is set by default,
and apparently Python defaults not only the terminal encoding to the locale
but that of every text file. So without LOCALE environment it defaults text
file encoding to ASCII. This causes problems with e.g. bitcoin.conf.

Luckily the locale doesn't affect the default encoding for str.encode() and
bytes.decode() on Python 3, so this is the only change necessary.