Add Gemfile and fix tests for Ruby 4.x compatibility

- Add Gemfile with test dependencies (minitest,csv, gruff, etc.)
- Pin minitest to version 5.x to avoid breaking changes introduced in minitest 6.x
- Require minitest/mock in test_helper.rb for stub method
- Update CI to use bundler-cache and bundle exec

Author: st0012

.github/workflows/test.yml

@@ -33,9 +33,10 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
 
       - name: Run tests
-        run: rake test
+        run: bundle exec rake test
 
   benchmark-default:
     runs-on: ubuntu-latest

Gemfile

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "csv"
+gem "gruff"
+gem "minitest", "~> 5.0"
+gem "rake"
+gem "rmagick"

Gemfile.lock

@@ -0,0 +1,42 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    bigdecimal (4.0.1)
+    csv (3.3.5)
+    gruff (0.29.0)
+      bigdecimal (>= 3.0)
+      histogram
+      rmagick (>= 5.5)
+    histogram (0.2.4.1)
+    minitest (5.27.0)
+    observer (0.1.2)
+    pkg-config (1.6.5)
+    rake (13.3.1)
+    rmagick (6.1.5)
+      observer (~> 0.1)
+      pkg-config (~> 1.4)
+
+PLATFORMS
+  arm64-darwin-24
+  ruby
+
+DEPENDENCIES
+  csv
+  gruff
+  minitest (~> 5.0)
+  rake
+  rmagick
+
+CHECKSUMS
+  bigdecimal (4.0.1)
+  csv (3.3.5) sha256=6e5134ac3383ef728b7f02725d9872934f523cb40b961479f69cf3afa6c8e73f
+  gruff (0.29.0)
+  histogram (0.2.4.1)
+  minitest (5.27.0) sha256=2d3b17f8a36fe7801c1adcffdbc38233b938eb0b4966e97a6739055a45fa77d5
+  observer (0.1.2) sha256=d8a3107131ba661138d748e7be3dbafc0d82e732fffba9fccb3d7829880950ac
+  pkg-config (1.6.5) sha256=33f9f81c5322983d22b439b8b672f27777b406fea23bfec74ff14bbeb42ec733
+  rake (13.3.1) sha256=8c9e89d09f66a26a01264e7e3480ec0607f0c497a861ef16063604b1b08eb19c
+  rmagick (6.1.5) sha256=91c1734cc0effc3b1be18e6705ea0353d73cb492547002f542763fa08445acd1
+
+BUNDLED WITH
+  4.1.0.dev

test/test_helper.rb

@@ -1 +1,2 @@
 require 'minitest/autorun'
+require 'minitest/mock'