rebase; extract factories and the mocks they are dependent on to

libraries/shared; adjust omni test_helpers to drop and recreate
checked_headers table to avoid reaching 1600 column limit after repeated
tests (dropping columns doesn't actually remove them from contributing
to that limit)

pkg/plugin/helpers/helpers.go

@@ -62,18 +62,16 @@ func CopyFile(src, dst string) error {
 	if err != nil {
 		return err
 	}
+	defer in.Close()
 	out, err := os.OpenFile(dst, syscall.O_CREAT|syscall.O_EXCL|os.O_WRONLY, os.FileMode(0666)) // Doesn't overwrite files
 	if err != nil {
-		in.Close()
 		return err
 	}
+	defer out.Close()
 
 	_, err = io.Copy(out, in)
 	if err != nil {
-		in.Close()
-		out.Close()
 		return err
 	}
-	in.Close()
 	return out.Close()
 }