Run fsync tests on mutable filesystem.

The purpose of those tests is to ensure fsync succeeds for
directories. However, they try to call fsync for / (the root
filesystem) that is usually mounted read-only, which does not
make sense because the whole point of fsync is to flush
uncommitted changes to the filesystem.

In fact, some well-known read-only filesystems (e.g. squashfs)
does not support fsync, so these tests fail if we use such
filesystems as the root filesystem.

This patch changes the tests to call fsync against /data
instead. /data is a user data partition and should be always
mounted read-write.

Bug: 28681455

(cherry picked from commit cfa3262ce46fdec81cb011ec78273df68c325c9c)

Change-Id: I3e50f7b6ef07e96138711eef1f678f90cc9eb632
diff --git a/tests/unistd_test.cpp b/tests/unistd_test.cpp
index 841641f..fae28a6 100644
--- a/tests/unistd_test.cpp
+++ b/tests/unistd_test.cpp
@@ -397,7 +397,7 @@
   close(fd);
 
   // The fd can even be a directory.
-  ASSERT_NE(-1, fd = open("/", O_RDONLY));
+  ASSERT_NE(-1, fd = open("/data", O_RDONLY));
   EXPECT_EQ(0, fn(fd));
   close(fd);