fix system test

Author: Linchin

google/cloud/bigquery/enums.py

@@ -482,7 +482,7 @@ class SourceColumnMatch(str, enum.Enum):
     columns to match the field names in the schema."""
 
 
-class TimestampPrecision(enum.Enum):
+class TimestampPrecision(object):
     """Precision (maximum number of total digits in base 10) for seconds of
     TIMESTAMP type."""
 

google/cloud/bigquery/schema.py

@@ -390,7 +390,7 @@ def timestamp_precision(self):
         """Union[enums.TimestampPrecision, int, None]: Precision (maximum number
         of total digits in base 10) for seconds of TIMESTAMP type.
         """
-        return self._properties.get("timestampPrecision")
+        return _helpers._int_or_none(self._properties.get("timestampPrecision"))
 
     def to_api_repr(self) -> dict:
         """Return a dictionary representing this schema field.