replace deprecated FunctionDefine in chat_test.go

[ryomak]

Hello,
I have made modifications to chat_test.go as it was using deprecated FunctionDefine.

[codecov]

Codecov Report

Merging #416 (91d2fcd) into master (581f70b) will not change coverage.
The diff coverage is n/a.

❗ Current head 91d2fcd differs from pull request most recent head 1190ab3. Consider uploading reports for the commit 1190ab3 to get more accurate results

@@           Coverage Diff           @@
##           master     #416   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files          17       17           
  Lines         690      690           
=======================================
  Hits          669      669           
  Misses         15       15           
  Partials        6        6           

[vvatanabe]

@ryomak Thanks for your contribution to the tests!