Coverage: Add specs for Condition#applicable? and Condition#level

`#level` will be deprecated soon, but Codecov still complains.

Author: mamhoff

promotions/spec/models/solidus_promotions/condition_spec.rb

@@ -14,6 +14,14 @@ def self.model_name
       def eligible?(_promotable, _options = {})
         true
       end
+
+      def applicable?(_promotable)
+        true
+      end
+
+      def level
+        :line_item
+      end
     end
   end
 
@@ -37,6 +45,16 @@ def eligible?(_promotable, _options = {})
     expect { test_condition_class.new.eligible?("promotable") }.not_to raise_error
   end
 
+  it "forces developer to implement #applicable?" do
+    expect { bad_test_condition_class.new.applicable?("promotable") }.to raise_error NotImplementedError
+    expect { test_condition_class.new.applicable?("promotable") }.not_to raise_error
+  end
+
+  it "forces developer to implement #level", :silence_deprecations do
+    expect { bad_test_condition_class.new.level }.to raise_error NotImplementedError
+    expect { test_condition_class.new.level }.not_to raise_error
+  end
+
   it "validates unique conditions for a promotion benefit" do
     # Because of Rails' STI, we can't use the anonymous class here
     promotion = create(:solidus_promotion, :with_adjustable_benefit)