fix tests -Wmultistatement-macros

newer gcc (9):
test_private.h:329:5: warning: macro expands to multiple statements [-Wmultistatement-macros]
     ind = strcmp((s1), (s2));                                                  \
     ^~~
test_gets_s.c:81:10: note: in expansion of macro 'EXPSTR'
     else EXPSTR(sub, "1234");
          ^~~~~~
test_gets_s.c:81:5: note: some parts of macro expansion are not guarded by this 'else' clause
     else EXPSTR(sub, "1234");