Add a test asserting that default fstab has /data entry

When aosp/1149619 is merged, userspace reboot will start to depend on
the fact that default fstab has an entry for /data, hence this test
to make sure that this is the case.

Bug: 135984674
Test: atest CtsFsMgrTestCases
Change-Id: I7a3c181a6b27a7b4dfc344ff04be712f3fe3376e

fs_mgr/tests/fs_mgr_test.cpp

@@ -1081,3 +1081,10 @@ source none5       swap   defaults      zram_backing_dev_path=/dev/path2
     EXPECT_EQ("none5", entry->mount_point);
     EXPECT_EQ("/dev/path2", entry->zram_backing_dev_path);
 }
+
+TEST(fs_mgr, DefaultFstabContainsUserdata) {
+    Fstab fstab;
+    ASSERT_TRUE(ReadDefaultFstab(&fstab)) << "Failed to read default fstab";
+    ASSERT_NE(nullptr, GetEntryForMountPoint(&fstab, "/data"))
+            << "Default fstab doesn't contain /data entry";
+}