增加buffer模块from接口XTS

https://gitee.com/openharmony/xts_acts/issues/I7M6T3Signed-off-by: NzWX1234017 <zhaoduwei3@huawei.com>

commonlibrary/ets_utils/buffer_lib_standard/src/main/js/test/Buffer.test.js

@@ -1912,6 +1912,18 @@ describe('BufferTest', function () {
     expect(str).assertEqual("this is a test");
   });
 
+  /**
+   * @tc.name: testfrom0269
+   * @tc.desc: Create a new buffer containing a newline character string. Creating using base64 encoding
+   * For example: buffer.from(string, encoding);
+   */
+  it("testfrom0269", 0, function () {
+    const str = `MIIEvwIBADANBgkqhkiG9w0BAQEFAASCBKkwggSlAgEAAoIBAQC/OgIQLaulKklB\npZltSaWwM3Cnzcmvh+LoqYFYjCePUxnoJRDusFqy4sjwBx9fn/XSxxJ9A0KxMtXa\nr7YFjwQo3FsIjLZ3+8wS+Kydcg==`;
+    const buf = buffer.from(str, 'base64');
+    let ref = buf.readInt8(buf.length - 1);
+    expect(ref).assertEqual(114);
+  });
+  
   /**
    * @tc.name: testBlobConstructor0270
    * @tc.desc: Creates a new Blob object containing a concatenation of the given sources.