chore: use .svg.txt extension on plugin test files (#1917)

.gitignore

@@ -13,4 +13,5 @@ test/regression-diffs
 test/cli/output
 coverage
 .DS_Store
+.vscode
 *.log

test/plugins/_index.test.js

@@ -5,7 +5,7 @@ import { fileURLToPath } from 'url';
 import { optimize } from '../../lib/svgo.js';
 
 const regEOL = new RegExp(EOL, 'g');
-const regFilename = /^(.*)\.(\d+)\.svg$/;
+const regFilename = /^(.*)\.(\d+)\.svg\.txt$/;
 const __dirname = PATH.dirname(fileURLToPath(import.meta.url));
 
 describe('plugins tests', function () {