Refactor: libcrmcommon: Standardize pcmk__xe_get_bool_attr return codes

Use the same return code scheme as other XML attribute getter functions.

Also, in the unit tests, simply assert not equal to pcmk_rc_ok when the
value is unparsable as a boolean. The specific return code seems more
like a detail of pcmk__parse_bool().

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>