Support minitest 6 where possible, pin < 6 elsewhere

Minitest 6 extracted minitest/mock into a separate gem (minitest-mock),
which breaks tests that require "minitest/mock".

- Rails 7.0, 7.1: pin minitest < 6 (these Rails versions don't
  constrain minitest themselves)
- Rails 7.2: already pins minitest < 6 in its own gemspec (rails/rails@6ea60702)
- Rails 8.0, 8.1, root Gemfile: use minitest 6 with minitest-mock gem

Author: flavorjones

.github/workflows/ci.yml

@@ -14,9 +14,12 @@ jobs:
           - "7.1"
           - "7.2"
           - "8.0"
+          - "8.1"
         exclude:
           - ruby-version: "3.1"
             rails-version: "8.0"
+          - ruby-version: "3.1"
+            rails-version: "8.1"
       fail-fast: false
     env:
       BUNDLE_GEMFILE: gemfiles/Gemfile.rails-${{ matrix.rails-version }}

Gemfile

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gemspec
 
 gem "rails", ">= 7.0.1"
+gem "minitest-mock"
 gem "rake"
 gem "debug"

gemfiles/Gemfile.rails-7.0

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gemspec path: ".."
 
 gem "rails", github: "rails/rails", branch: "7-0-stable"
+gem "minitest", "< 6"
 gem "rake"
 gem "debug"

gemfiles/Gemfile.rails-7.1

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gemspec path: ".."
 
 gem "rails", "~> 7.1.0"
+gem "minitest", "< 6"
 gem "rake"
 gem "debug"

gemfiles/Gemfile.rails-7.2

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gemspec path: ".."
 
 gem "rails", "~> 7.2.0"
+gem "minitest-mock"
 gem "rake"
 gem "debug"

gemfiles/Gemfile.rails-8.0

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 gemspec path: ".."
 
 gem "rails", "~> 8.0"
+gem "minitest-mock"
 gem "rake"
 gem "debug"

gemfiles/Gemfile.rails-8.1

@@ -0,0 +1,8 @@
+source "https://rubygems.org"
+
+gemspec path: ".."
+
+gem "rails", "~> 8.1"
+gem "minitest-mock"
+gem "rake"
+gem "debug"