Fix an error for Minitst/AssertOperator and Minitest/RefuteOperator

This PR fixes the following error for `Minitst/AssertOperator` and `Minitest/RefuteOperator`
when running with Ruby 2.7:

```console
$ ruby -v
ruby 2.7.8p225 (2023-03-30 revision 1f4d455848) [x86_64-darwin19]

$ bundle exec rake

Finished in 2.522134s, 15.0666 runs/s, 18.6350 assertions/s.

  1) Error: RefuteOperatorTest#test_registers_offense_when_using_refute_with_operator_method_and_message:
FrozenError: can't modify frozen String: "expected, :<, actual"
    /Users/koic/src/github.com/rubocop/rubocop-minitest/lib/rubocop/cop/minitest/refute_operator.rb:46:
    in `build_new_arguments'
    /Users/koic/src/github.com/rubocop/rubocop-minitest/lib/rubocop/cop/minitest/refute_operator.rb:30:
    in `on_send'
```

And it introduces CI for the oldest RuboCop version still supported,
implementing a workflow for regression testing that is similar to the one found a
rubocop/rubocop-rails@c9acb7a.

.github/workflows/test.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  oldest_supported_rubocop:
+    runs-on: ubuntu-latest
+    name: The oldest supported RuboCop version
+    steps:
+      - uses: actions/checkout@v4
+      - name: Use the oldest supported RuboCop
+        run: |
+          sed -e "/gem 'rubocop', github: 'rubocop\/rubocop'/d" \
+              -e "/gem 'rubocop-performance',/d" -i Gemfile
+          cat << EOF > Gemfile.local
+            gem 'rubocop', '1.39.0' # Specify the oldest supported RuboCop version
+          EOF
+      - name: set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+          bundler-cache: true
+      - name: test
+        run: bundle exec rake

changelog/fix_error_for_minitest_assert_operator_and_refute_operator.md

@@ -0,0 +1,2 @@
+* [#275](https://github.com/rubocop/rubocop-minitest/issues/275): Make `Minitest/AssertMatch` aware of `assert_operator`
+when running with Ruby 2.7. ([@koic][])

lib/rubocop/cop/minitest/assert_operator.rb

@@ -40,7 +40,7 @@ def on_send(node)
 
         def build_new_arguments(node)
           lhs, op, rhs = *node.first_argument
-          new_arguments = "#{lhs.source}, :#{op}, #{rhs.source}"
+          new_arguments = +"#{lhs.source}, :#{op}, #{rhs.source}"
 
           if node.arguments.count == 2
             new_arguments << ", #{node.last_argument.source}"

lib/rubocop/cop/minitest/refute_operator.rb

@@ -40,7 +40,7 @@ def on_send(node)
 
         def build_new_arguments(node)
           lhs, op, rhs = *node.first_argument
-          new_arguments = "#{lhs.source}, :#{op}, #{rhs.source}"
+          new_arguments = +"#{lhs.source}, :#{op}, #{rhs.source}"
 
           if node.arguments.count == 2
             new_arguments << ", #{node.last_argument.source}"