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

[eugenis]

LGTM

lib/msan/tests/msan_test.cc

81

Does not look very reliable, but ok.

lib/msan/tests/msan_test.cc

 # include <immintrin.h>
 #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"
 #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"
 #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, DISABLED_ioctl) {
   struct winsize ws;
   EXPECT_EQ(ioctl(2, TIOCGWINSZ, &ws), 0);
   EXPECT_NOT_POISONED(ws.ws_col);
 }
 
 TEST(MemorySanitizer, readlink) {
   char *x = new char[1000];
-  readlink("/proc/self/exe", x, 1000);
+  readlink(SYMLINK_TO_READ, x, 1000);
   EXPECT_NOT_POISONED(x[0]);
   delete [] x;
 }
 
 TEST(MemorySanitizer, stat) {
   struct stat* st = new struct stat;
   int res = stat(FILE_TO_READ, st);
   ASSERT_EQ(0, res);