Add shared example for database_cleaner adapter

lib/database_cleaner/spec/shared_examples.rb

@@ -1,15 +1,24 @@
-RSpec.shared_examples_for "a generic strategy" do
+RSpec.shared_examples_for "a database_cleaner strategy" do
   it { is_expected.to respond_to(:db)  }
-end
-
-RSpec.shared_examples_for "a generic truncation strategy" do
+  it { is_expected.to respond_to(:db=)  }
   it { is_expected.to respond_to(:start) }
   it { is_expected.to respond_to(:clean) }
   it { is_expected.to respond_to(:cleaning) }
 end
 
-RSpec.shared_examples_for "a generic transaction strategy" do
-  it { is_expected.to respond_to(:start) }
-  it { is_expected.to respond_to(:clean) }
-  it { is_expected.to respond_to(:cleaning) }
+RSpec.shared_examples_for "a database_cleaner adapter" do
+  it { expect(described_class).to respond_to(:available_strategies) }
+  it { expect(described_class).to respond_to(:default_strategy) }
+
+  it 'default_strategy should be part of available_strategies' do
+    expect(described_class.available_strategies).to include(described_class.default_strategy)
+  end
+
+  describe 'all strategies should adhere to a database_cleaner strategy interface' do
+    described_class.available_strategies.each do |strategy|
+      subject { described_class.const_get(strategy.to_s.capitalize).new }
+
+      it_behaves_like 'a database_cleaner strategy'
+    end
+  end
 end