Use test-unit gem instead of test-framework of ruby repo (#216)

Co-authored-by: Sutou Kouhei <kou@clear-code.com>

Author: hsbt

.github/workflows/test.yml

@@ -85,6 +85,7 @@ jobs:
       - name: Install
         run: |
           rake install
+          gem install test-unit
       - name: Test
         run: |
           mkdir -p tmp

csv.gemspec

@@ -60,4 +60,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "benchmark_driver"
+  spec.add_development_dependency "test-unit", ">= 3.4.3"
 end