data: simplify dispatching provider tests (#3858)

Summary:
Per comment of @nfelt on #3847, we can still test for delegate equality:
<#3847 (comment)>

wchargin-branch: data-dispatch-test-bare

Author: wchargin

tensorboard/data/dispatching_provider_test.py

@@ -287,14 +287,8 @@ def test_blob_sequences_prefixed(self):
         self.assertEqual(listing, expected_listing)
 
         blobs = self._get_blobs(self.with_unpfx, "bar:a:b:c")
-        self.assertEqual(
-            blobs,
-            {
-                "a:b:c/test": {
-                    "input.images": [[b"experiment: a:b:c", b"name: Bar"]]
-                }
-            },
-        )
+        expected_blobs = self._get_blobs(self.bar_provider, "a:b:c")
+        self.assertEqual(blobs, expected_blobs)
 
     def test_blob_sequences_unprefixed(self):
         listing = self.with_unpfx.list_blob_sequences(
@@ -306,14 +300,8 @@ def test_blob_sequences_unprefixed(self):
         self.assertEqual(listing, expected_listing)
 
         blobs = self._get_blobs(self.with_unpfx, "baz")
-        self.assertEqual(
-            blobs,
-            {
-                "baz/test": {
-                    "input.images": [[b"experiment: baz", b"name: BAZ"]]
-                }
-            },
-        )
+        expected_blobs = self._get_blobs(self.baz_provider, "baz")
+        self.assertEqual(blobs, expected_blobs)
 
     def test_blobs_error_cases(self):
         with self.assertRaisesRegex(