test: string-decorater.lastChar

Added test for string-decorater.lastChar to improve coverage. PR-URL: #21084 Reviewed-By: Tiancheng "Timothy" Gu <[email protected]> Reviewed-By: Lance Ball <[email protected]> Reviewed-By: Trivikram Kamat <[email protected]>

Author: Masashi Hirano

test/parallel/test-string-decoder.js

@@ -90,7 +90,8 @@ test('utf16le', Buffer.from('3DD84DDC', 'hex'), '\ud83d\udc4d'); // thumbs up
 decoder=newStringDecoder('utf8');
 assert.strictEqual(decoder.write(Buffer.from('E1','hex')),'');
 
-// A quick test for lastNeed & lastTotal which are undocumented.
+// A quick test for lastChar, lastNeed & lastTotal which are undocumented.
+assert(decoder.lastChar.equals(newUint8Array([0xe1,0,0,0])));
 assert.strictEqual(decoder.lastNeed,2);
 assert.strictEqual(decoder.lastTotal,3);