test-condition: fix test_condition_test_group() (#6531)

test-condition: fix test_condition_test_group() (#6531)

I hit a test failure with the `max_gid+1` test.  Problem is that we loop
over 0..r, but set `r` again within the loop (to 1).  So max_gid is only
set based on the first supplementary GID.

ConditionGroup=1000 → 1
ConditionGroup=4 → 1
ConditionGroup=adm → 1
ConditionGroup=1001 → 1
Assertion 'r == 0' failed at ../src/test/test-condition.c:462, function
test_condition_test_group(). Aborting.

$ id
uid=1000(alan-sysop) gid=1000(alan-sysop) groups=1000(alan-sysop),4(adm),
10(wheel),1001(sshlogin)