[chore][cmd/mdatagen] Properly test bytes attribute type (open-teleme…

…try#30106)

**Description:** 
Changes how attributes of type `bytes` are tested, as they were being
initialized incorrectly before.

**Link to tracking Issue:** Resolves open-telemetry#29923

**Testing:** 
Fake attribute added for testing:
```
attributes:
  # Added a fake attribute in a metadata.yaml file
  fake:
    description: "Fake attribute for testing"
    type: bytes
```
Test case generated by `cmd/mdatagen`:
```
                                       attrVal, ok = dp.Attributes().Get("fake")
                                       assert.True(t, ok)
                                       assert.EqualValues(t, []byte("fake-val"), attrVal.Bytes())
```

cmd/mdatagen/loader.go

@@ -194,7 +194,7 @@ func (a attribute) TestValue() string {
 	case pcommon.ValueTypeSlice:
 		return fmt.Sprintf(`[]any{"%s-item1", "%s-item2"}`, a.FullName, a.FullName)
 	case pcommon.ValueTypeBytes:
-		return fmt.Sprintf(`bytes("%s-val")`, a.FullName)
+		return fmt.Sprintf(`[]byte("%s-val")`, a.FullName)
 	}
 	return ""
 }