Refactor Prometheus::AlertPresenter specs

Refactor AlertPresenter specs to check an actual
markdown instead of included parts.

ee/spec/presenters/projects/prometheus/alert_presenter_spec.rb

@@ -41,9 +41,14 @@ describe Projects::Prometheus::AlertPresenter do
 
     context 'without default payload' do
       it do
-        is_expected.to include('## Summary')
-        is_expected.to include('* starts_at:')
-        is_expected.not_to include('* full_query:')
+        is_expected.to eq(
+          <<~MARKDOWN.chomp
+            ## Summary
+
+            * starts_at: #{presenter.starts_at}
+
+          MARKDOWN
+        )
       end
     end
 
@@ -53,8 +58,15 @@ describe Projects::Prometheus::AlertPresenter do
       end
 
       it do
-        is_expected.to include('* foo: value1')
-        is_expected.to include('* bar: value2')
+        is_expected.to eq(
+          <<~MARKDOWN.chomp
+            ## Summary
+
+            * starts_at: #{presenter.starts_at}
+            * foo: value1
+            * bar: value2
+          MARKDOWN
+        )
       end
     end
 
@@ -63,7 +75,17 @@ describe Projects::Prometheus::AlertPresenter do
         payload['generatorURL'] = 'http://host?g0.expr=query'
       end
 
-      it { is_expected.to include('* full_query: `query`') }
+      it do
+        is_expected.to eq(
+          <<~MARKDOWN.chomp
+            ## Summary
+
+            * starts_at: #{presenter.starts_at}
+            * full_query: `query`
+
+          MARKDOWN
+        )
+      end
     end
   end