Added a convention that sentinel errors go near the top of the file, and applied it

internal/cli/migrate/common_test.go

@@ -10,6 +10,9 @@ import (
 	"github.com/pressly/goose/v3"
 )
 
+// errWrite is the error failingWriter fails every write with.
+var errWrite = errors.New("write failed")
+
 // The commands are tested through the real command tree in cmd/jobsearch.
 // These tests cover reporting a failed migration, which a working set of
 // migrations can't trigger.
@@ -92,8 +95,6 @@ func TestPrintResults(t *testing.T) {
 // failingWriter fails every write.
 type failingWriter struct{}
 
-var errWrite = errors.New("write failed")
-
 func (failingWriter) Write([]byte) (int, error) { return 0, errWrite }
 
 // A write error is returned, and the failed migration is still reported.