Add RSpectre to CI

- This can help detect unused test setup on an ongoing basis and also helps me by giving me a bigger corpus of [`rspectre`](https://github.com/dgollahon/rspectre) users.

.github/workflows/ci.yml

@@ -149,3 +149,21 @@ jobs:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
     - run: bundle exec rubocop
+  rspectre:
+    name: RSpectre
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 10
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [ruby-3.2]
+        os: [ubuntu-latest]
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/Gemfile.rspectre
+    steps:
+    - uses: actions/checkout@v3
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - run: bundle exec rspectre

Gemfile.lock

@@ -20,7 +20,7 @@ GEM
     diff-lcs (1.5.0)
     json (2.6.3)
     parallel (1.22.1)
-    parser (3.2.2.0)
+    parser (3.2.2.1)
       ast (~> 2.4.1)
     rainbow (3.1.1)
     regexp_parser (2.6.2)

Gemfile.rspectre

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
+
+gemspec name: 'mutant'
+
+# This must be specified in a separate Gemfile because `rspectre` does not support ruby 2.7
+gem 'rspectre', '~> 0.1'
+
+eval_gemfile 'Gemfile.shared'

Gemfile.rspectre.lock

@@ -0,0 +1,80 @@
+PATH
+  remote: .
+  specs:
+    mutant (0.11.19)
+      diff-lcs (~> 1.3)
+      parser (~> 3.2.2)
+      regexp_parser (~> 2.6.1)
+      sorbet-runtime (~> 0.5.0)
+      unparser (~> 0.6.7)
+
+GEM
+  remote: https://oss:Px2ENN7S91OmWaD5G7MIQJi1dmtmYrEh@gem.mutant.dev/
+  specs:
+    mutant-license (0.1.1.2.2355046999240944981729280251890364410689.5)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    ast (2.4.2)
+    diff-lcs (1.5.0)
+    json (2.6.3)
+    parallel (1.23.0)
+    parser (3.2.2.1)
+      ast (~> 2.4.1)
+    rainbow (3.1.1)
+    regexp_parser (2.6.2)
+    rexml (3.2.5)
+    rspec (3.12.0)
+      rspec-core (~> 3.12.0)
+      rspec-expectations (~> 3.12.0)
+      rspec-mocks (~> 3.12.0)
+    rspec-core (3.12.2)
+      rspec-support (~> 3.12.0)
+    rspec-expectations (3.12.3)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.12.0)
+    rspec-its (1.3.0)
+      rspec-core (>= 3.0.0)
+      rspec-expectations (>= 3.0.0)
+    rspec-mocks (3.12.5)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.12.0)
+    rspec-support (3.12.0)
+    rspectre (0.1.0)
+      parser (>= 3.2.2.1)
+      rspec (~> 3.9)
+    rubocop (1.51.0)
+      json (~> 2.3)
+      parallel (~> 1.10)
+      parser (>= 3.2.0.0)
+      rainbow (>= 2.2.2, < 4.0)
+      regexp_parser (>= 1.8, < 3.0)
+      rexml (>= 3.2.5, < 4.0)
+      rubocop-ast (>= 1.28.0, < 2.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 2.4.0, < 3.0)
+    rubocop-ast (1.28.1)
+      parser (>= 3.2.1.0)
+    ruby-progressbar (1.13.0)
+    sorbet-runtime (0.5.10837)
+    unicode-display_width (2.4.2)
+    unparser (0.6.7)
+      diff-lcs (~> 1.3)
+      parser (>= 3.2.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  mutant!
+  mutant-license!
+  parallel (~> 1.3)
+  rspec (~> 3.10)
+  rspec-core (~> 3.10)
+  rspec-its (~> 1.3.0)
+  rspectre (~> 0.1)
+  rubocop (~> 1.7)
+
+BUNDLED WITH
+   2.4.13