[sanitizer][test] Print result in getpwnam_r_invalid_user.cpp (#162217)
This test has been flaky failing on sanitizer-ppc64le-linux since at
least 9 days ago
(https://lab.llvm.org/buildbot/#/builders/72/builds/15257), but the
exact cause is unclear because the only output is that `assert(res == 0
|| res == ENOENT);` failed.
To aid debugging, this patch prints out the result of the `getpwnam_r`
call.