tm_ostream opened with file is not tested

[jingkaimori]

Lolly Version

master

Operating System Version and Architecture

Windows 22H2

Describe Bug

All tests in tm_ostream_test.cpp build a stream writing to internal string, with no testcase writing into file.

Expected Behavior

• test tm_ostream opened on file, or remove this feature;
• test read and write on utf-8 filename
• test interoperability between tm_ostream, load_string, and save_string