test: unset 'xpg_echo' bash shell option

test: unset 'xpg_echo' bash shell option

When 'xpg_echo' bash shell option is unset (usually the default)
echo builtin does not expand backslash-escape sequences by default
(i.e. '\n' is echoed as '\n' instead of newline). Not all bash
installations have this feature we depend on activated by default.

Note that the feature is bash (and GNU /bin/echo) specific. It is used
as it is convenient. If portability is needed (elsewhere) use printf(1)
(also often available as a shell builtin).