Add test: check column DEFAULTs preserved on set NOT NULL (#235)

Add a check to ensure that any `DEFAULT` on a column that is set to `NOT
NULL` is preserved by the set `NOT NULL` operation.

This already works as a consequence of earlier PRs for
#227 but it's worth explicitly
testing this behaviour.

pkg/migrations/op_set_notnull_test.go

@@ -299,6 +299,70 @@ func TestSetNotNull(t *testing.T) {
 				ConstraintMustExist(t, db, "public", "employees", "fk_employee_department")
 			},
 		},
+		{
+			name: "setting a nullable column to not null retains any default defined on the column",
+			migrations: []migrations.Migration{
+				{
+					Name: "01_add_table",
+					Operations: migrations.Operations{
+						&migrations.OpCreateTable{
+							Name: "users",
+							Columns: []migrations.Column{
+								{
+									Name: "id",
+									Type: "integer",
+									Pk:   true,
+								},
+								{
+									Name:     "name",
+									Type:     "text",
+									Nullable: true,
+									Default:  ptr("'anonymous'"),
+								},
+							},
+						},
+					},
+				},
+				{
+					Name: "02_set_not_null",
+					Operations: migrations.Operations{
+						&migrations.OpAlterColumn{
+							Table:    "users",
+							Column:   "name",
+							Nullable: ptr(false),
+							Up:       "(SELECT CASE WHEN name IS NULL THEN 'anonymous' ELSE name END)",
+						},
+					},
+				},
+			},
+			afterStart: func(t *testing.T, db *sql.DB) {
+				// A row can be inserted into the new version of the table.
+				MustInsert(t, db, "public", "02_set_not_null", "users", map[string]string{
+					"id": "1",
+				})
+
+				// The newly inserted row respects the default value of the column.
+				rows := MustSelect(t, db, "public", "02_set_not_null", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "name": "anonymous"},
+				}, rows)
+			},
+			afterRollback: func(t *testing.T, db *sql.DB) {
+			},
+			afterComplete: func(t *testing.T, db *sql.DB) {
+				// A row can be inserted into the new version of the table.
+				MustInsert(t, db, "public", "02_set_not_null", "users", map[string]string{
+					"id": "2",
+				})
+
+				// The newly inserted row respects the default value of the column.
+				rows := MustSelect(t, db, "public", "02_set_not_null", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "name": "anonymous"},
+					{"id": 2, "name": "anonymous"},
+				}, rows)
+			},
+		},
 	})
 }