[Msan] Fix the getgrnam_r unit test to pass on FreeBSD

[eugenis]

LGTM

lib/msan/tests/msan_test.cc

 #endif
 
 // On FreeBSD procfs is not enabled by default.
 #if defined(__FreeBSD__)
 # define FILE_TO_READ "/bin/cat"
 # define DIR_TO_READ "/bin"
 # define SUBFILE_TO_READ "cat"
 # define SYMLINK_TO_READ "/usr/bin/tar"
+# define SUPERUSER_GROUP "wheel"
 #else
 # define FILE_TO_READ "/proc/self/stat"
 # define DIR_TO_READ "/proc/self"
 # define SUBFILE_TO_READ "stat"
 # define SYMLINK_TO_READ "/proc/self/exe"
+# define SUPERUSER_GROUP "root"
 #endif
 
 static const size_t kPageSize = 4096;
 
 typedef unsigned char      U1;
 typedef unsigned short     U2;  // NOLINT
 typedef unsigned int       U4;
 typedef unsigned long long U8;  // NOLINT
TEST(MemorySanitizer, getpwnam_r_positive) {
   int res;
   EXPECT_UMR(res = getpwnam_r(s, &pwd, buf, sizeof(buf), &pwdres));
 }
 
 TEST(MemorySanitizer, getgrnam_r) {
   struct group grp;
   struct group *grpres;
   char buf[10000];
-  int res = getgrnam_r("root", &grp, buf, sizeof(buf), &grpres);
+  int res = getgrnam_r(SUPERUSER_GROUP, &grp, buf, sizeof(buf), &grpres);
   ASSERT_EQ(0, res);
+  // Note that getgrnam_r() returns 0 if the matching group is not found.
+  ASSERT_NE(nullptr, grpres);
   EXPECT_NOT_POISONED(grp.gr_name);
   ASSERT_TRUE(grp.gr_name != NULL);
   EXPECT_NOT_POISONED(grp.gr_name[0]);
   EXPECT_NOT_POISONED(grp.gr_gid);
   EXPECT_NOT_POISONED(grpres);
 }
 
 TEST(MemorySanitizer, getpwent) {