Use smaller testdata files (#1283)

### Changelog
None

### Docs
None

### Description
Use smaller and simpler `demo.bag` and `demo.mcap` files.

Fixes: FG-9552

go/mcap/reader_test.go

@@ -445,7 +445,7 @@ func TestReaderCounting(t *testing.T) {
 				require.NoError(t, err)
 				c++
 			}
-			assert.Equal(t, 1606, c)
+			assert.Equal(t, 3, c)
 		})
 	}
 }
@@ -673,7 +673,7 @@ func TestReadingDiagnostics(t *testing.T) {
 		require.NoError(t, err)
 		c++
 	}
-	assert.Equal(t, 52, c)
+	assert.Equal(t, 1, c)
 }
 
 func TestReadingMetadata(t *testing.T) {

python/mcap/tests/test_reader.py

@@ -67,7 +67,7 @@ def test_all_messages(reader_cls: AnyReaderSubclass):
             assert isinstance(message, Message)
             count += 1
 
-        assert count == 1606
+        assert count == 3
 
 
 @pytest.mark.parametrize("reader_cls", READER_SUBCLASSES)
@@ -76,8 +76,8 @@ def test_time_range(reader_cls: AnyReaderSubclass):
     with open(DEMO_MCAP, "rb") as f:
         reader: McapReader = reader_cls(f)
         count = 0
-        start = int(1490149582 * 1e9)
-        end = int(1490149586 * 1e9)
+        start = int(40)
+        end = int(43)
         for schema, channel, message in reader.iter_messages(
             start_time=start, end_time=end
         ):
@@ -88,7 +88,7 @@ def test_time_range(reader_cls: AnyReaderSubclass):
             assert message.log_time >= start
             count += 1
 
-        assert count == 825
+        assert count == 1
 
 
 @pytest.mark.parametrize("reader_cls", READER_SUBCLASSES)
@@ -104,7 +104,7 @@ def test_only_diagnostics(reader_cls: AnyReaderSubclass):
             assert isinstance(message, Message)
             count += 1
 
-        assert count == 52
+        assert count == 1
 
 
 def write_json_mcap(filepath: Path):