Fix Mocha deprecation warning

mishina2228 · mishina2228 · commit 5871b9386715 · 2022-10-22T14:54:15.000+09:00
```
Mocha deprecation warning at /.../net-scp/test/common.rb:2:in `require': Require 'mocha/test_unit', 'mocha/minitest' or 'mocha/api' instead of 'mocha/setup'.
```
diff --git a/test/common.rb b/test/common.rb
@@ -1,5 +1,5 @@
 require 'test/unit'
-require 'mocha/setup'
+require 'mocha/test_unit'
 
 begin
   gem 'net-ssh', ">= 2.0.0"
