tests/test-gendoc.out

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

% extracting documentation from C
checking for parse errors

% extracting documentation from da
checking for parse errors

% extracting documentation from de
checking for parse errors

% extracting documentation from el
checking for parse errors

% extracting documentation from fr
checking for parse errors

% extracting documentation from it
checking for parse errors

% extracting documentation from ja
checking for parse errors

% extracting documentation from pt_BR
checking for parse errors

% extracting documentation from sv
checking for parse errors

% extracting documentation from zh_CN
checking for parse errors

% extracting documentation from zh_TW
checking for parse errors