Fix cert_manager_spec to not modify String

Also break up long string to be readable

spec/models/clusters/applications/cert_manager_spec.rb

@@ -11,7 +11,27 @@ describe Clusters::Applications::CertManager do
   include_examples 'cluster application initial status specs'
 
   describe '#install_command' do
-    let(:cluster_issuer_file) { { "cluster_issuer.yaml": "---\napiVersion: certmanager.k8s.io/v1alpha1\nkind: ClusterIssuer\nmetadata:\n  name: letsencrypt-prod\nspec:\n  acme:\n    server: https://acme-v02.api.letsencrypt.org/directory\n    email: admin@example.com\n    privateKeySecretRef:\n      name: letsencrypt-prod\n    http01: {}\n" } }
+    let(:cert_email) { 'admin@example.com' }
+
+    let(:cluster_issuer_file) do
+      file_contents = <<~EOF
+      ---
+      apiVersion: certmanager.k8s.io/v1alpha1
+      kind: ClusterIssuer
+      metadata:
+        name: letsencrypt-prod
+      spec:
+        acme:
+          server: https://acme-v02.api.letsencrypt.org/directory
+          email: #{cert_email}
+          privateKeySecretRef:
+            name: letsencrypt-prod
+          http01: {}
+      EOF
+
+      { "cluster_issuer.yaml": file_contents }
+    end
+
     subject { cert_manager.install_command }
 
     it { is_expected.to be_an_instance_of(Gitlab::Kubernetes::Helm::InstallCommand) }
@@ -26,9 +46,10 @@ describe Clusters::Applications::CertManager do
     end
 
     context 'for a specific user' do
+      let(:cert_email) { 'abc@xyz.com' }
+
       before do
-        cert_manager.email = 'abc@xyz.com'
-        cluster_issuer_file[:'cluster_issuer.yaml'].gsub! 'admin@example.com', 'abc@xyz.com'
+        cert_manager.email = cert_email
       end
 
       it 'should use his/her email to register issuer with certificate provider' do