Fix tests of special devices.

Regular files were created, as the 'mode' argument of os.mknod was missing.
However, creating devices requires root; so we can't reasonably do that in
tests.

Instead, we're using /dev/null instead of creating one.
And while we're at it, let's also use /dev/zero (which, if not handled
properly, will result in an infinite read).

swh/model/tests/test_from_disk.py

@@ -102,7 +102,6 @@ class DataMixin:
 
         self.specials = {
             b'fifo': os.mkfifo,
-            b'devnull': lambda path: os.mknod(path, device=os.makedev(1, 3)),
         }
 
         self.empty_content = {
@@ -507,7 +506,7 @@ class FileToContent(DataMixin, unittest.TestCase):
         self.make_symlinks(self.tmpdir_name)
         self.make_specials(self.tmpdir_name)
 
-    def test_file_to_content(self):
+    def test_symlink_to_content(self):
         # Check whether loading the data works
         for data in [True, False]:
             for filename, symlink in self.symlinks.items():
@@ -515,16 +514,25 @@ class FileToContent(DataMixin, unittest.TestCase):
                 conv_content = Content.from_file(path=path, data=data)
                 self.assertContentEqual(conv_content, symlink, check_data=data)
 
+    def test_file_to_content(self):
+        for data in [True, False]:
             for filename, content in self.contents.items():
                 path = os.path.join(self.tmpdir_name, filename)
                 conv_content = Content.from_file(path=path, data=data)
                 self.assertContentEqual(conv_content, content, check_data=data)
 
+    def test_special_to_content(self):
+        for data in [True, False]:
             for filename in self.specials:
                 path = os.path.join(self.tmpdir_name, filename)
                 conv_content = Content.from_file(path=path, data=data)
                 self.assertContentEqual(conv_content, self.empty_content)
 
+            for path in ['/dev/null', '/dev/zero']:
+                path = os.path.join(self.tmpdir_name, filename)
+                conv_content = Content.from_file(path=path, data=data)
+                self.assertContentEqual(conv_content, self.empty_content)
+
     def test_symlink_max_length(self):
         for max_content_length in [4, 10]:
             for filename, symlink in self.symlinks.items():