tests: fix -Wmissing-variable-declarations warnings in vfs tests

```
../../../../tests/lib/vfs/vfs_parse_ls_lga.c:39:23: warning: no previous extern declaration for non-static variable 'test_subclass1' [-Wmissing-variable-declarations]
struct vfs_s_subclass test_subclass1;
                      ^
../../../../tests/lib/vfs/vfs_parse_ls_lga.c:42:21: warning: no previous extern declaration for non-static variable 'vfs_root_entry' [-Wmissing-variable-declarations]
struct vfs_s_entry *vfs_root_entry;
                    ^
../../../../tests/lib/vfs/vfs_s_get_path.c:40:23: warning: no previous extern declaration for non-static variable 'test_subclass1' [-Wmissing-variable-declarations]
struct vfs_s_subclass test_subclass1, test_subclass2, test_subclass3;
                      ^
```

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

tests/lib/vfs/vfs_parse_ls_lga.c

@@ -36,10 +36,10 @@
 #include "src/vfs/local/local.c"
 
 
-struct vfs_s_subclass test_subclass1;
+static struct vfs_s_subclass test_subclass1;
 static struct vfs_class *vfs_test_ops1 = VFS_CLASS (&test_subclass1);
 
-struct vfs_s_entry *vfs_root_entry;
+static struct vfs_s_entry *vfs_root_entry;
 static struct vfs_s_inode *vfs_root_inode;
 static struct vfs_s_super *vfs_test_super;
 

tests/lib/vfs/vfs_s_get_path.c

@@ -37,7 +37,7 @@
 #define ETALON_VFS_NAME "#test2:user:pass@host.net"
 #define ETALON_VFS_URL_NAME "test2://user:pass@host.net"
 
-struct vfs_s_subclass test_subclass1, test_subclass2, test_subclass3;
+static struct vfs_s_subclass test_subclass1, test_subclass2, test_subclass3;
 static struct vfs_class *vfs_test_ops1 = VFS_CLASS (&test_subclass1);
 static struct vfs_class *vfs_test_ops2 = VFS_CLASS (&test_subclass2);
 static struct vfs_class *vfs_test_ops3 = VFS_CLASS (&test_subclass3);