tests: ignore the return code of chmod in test-inherit-mode
In freebsd, a newly created directory has the same group as the parent
directory by default. That means that the test directory created by
test-inherit-mode.t is owned by root's group, so "chmod g+s .hg/store"
fails to set the SGID bit and returns 1. If we ignore chmod's return
code, the testsuite passes again.