support csv file fixtures for unit tests

.changes/unreleased/Features-20231106-194752.yaml

@@ -0,0 +1,6 @@
+kind: Features
+body: Add support of csv file fixtures to unit testing
+time: 2023-11-06T19:47:52.501495-06:00
+custom:
+  Author: emmyoop
+  Issue: "8290"

core/dbt/contracts/graph/unparsed.py

@@ -787,22 +787,40 @@
     def rows(self) -> Union[str, List[Dict[str, Any]]]:
         return []
 
+    @property
+    def fixture(self) -> Optional[str]:  # TODO: typing
+        return None
+
     def get_rows(self) -> List[Dict[str, Any]]:
         if self.format == UnitTestFormat.Dict:
             assert isinstance(self.rows, List)
             return self.rows
         elif self.format == UnitTestFormat.CSV:
-            assert isinstance(self.rows, str)
-            dummy_file = StringIO(self.rows)
-            reader = csv.DictReader(dummy_file)
-            rows = []
-            for row in reader:
-                rows.append(row)
+            if self.fixture:
+                # TODO: need to add logic to parse csv file into rows list. this is the exact logis as inline csv for now?
+                assert isinstance(self.fixture, str)
+                dummy_file = StringIO(self.fixture)
+                reader = csv.DictReader(dummy_file)
+                rows = []
+                for row in reader:
+                    rows.append(row)
+            else:  # using inline csv
+                assert isinstance(self.rows, str)
+                dummy_file = StringIO(self.rows)
+                reader = csv.DictReader(dummy_file)
+                rows = []
+                for row in reader:
+                    rows.append(row)
             return rows
 
     def validate_fixture(self, fixture_type, test_name) -> None:
-        if (self.format == UnitTestFormat.Dict and not isinstance(self.rows, list)) or (
-            self.format == UnitTestFormat.CSV and not isinstance(self.rows, str)
+        if self.format == UnitTestFormat.Dict and not isinstance(self.rows, list):
+            raise ParsingError(
+                f"Unit test {test_name} has {fixture_type} rows which do not match format {self.format}"
+            )
+
+        if self.format == UnitTestFormat.CSV and not (
+            isinstance(self.rows, str) or (self.fixture is not None)
         ):
             raise ParsingError(
                 f"Unit test {test_name} has {fixture_type} rows which do not match format {self.format}"
@@ -814,12 +832,14 @@
     input: str
     rows: Union[str, List[Dict[str, Any]]] = ""
     format: UnitTestFormat = UnitTestFormat.Dict
+    fixture: Optional[str] = None
 
 
 @dataclass
 class UnitTestOutputFixture(dbtClassMixin, UnitTestFixture):
     rows: Union[str, List[Dict[str, Any]]] = ""
     format: UnitTestFormat = UnitTestFormat.Dict
+    fixture: Optional[str] = None
 
 
 @dataclass