Update tests for snippets to support default branch

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/331906

spec/lib/gitlab/import_export/snippet_repo_restorer_spec.rb

@@ -29,7 +29,7 @@ RSpec.describe Gitlab::ImportExport::SnippetRepoRestorer do
           expect(restorer.restore).to be_truthy
         end.to change { SnippetRepository.count }.by(1)
 
-        blob = snippet.repository.blob_at('HEAD', snippet.file_name)
+        blob = snippet.repository.blob_at(snippet.default_branch, snippet.file_name)
         expect(blob).not_to be_nil
         expect(blob.data).to eq(snippet.content)
       end

spec/models/snippet_spec.rb

@@ -828,14 +828,10 @@ RSpec.describe Snippet do
       end
 
       context 'when default branch in settings is different from "master"' do
-        let(:default_branch) { 'main' }
+        let(:default_branch) { 'custom-branch' }
 
         it 'changes the HEAD reference to the default branch' do
-          expect(File.read(head_path).squish).to eq 'ref: refs/heads/master'
-
-          subject
-
-          expect(File.read(head_path).squish).to eq "ref: refs/heads/#{default_branch}"
+          expect { subject }.to change { File.read(head_path).squish }.to("ref: refs/heads/#{default_branch}")
         end
       end
     end

spec/requests/api/graphql/mutations/snippets/create_spec.rb

@@ -85,8 +85,8 @@ RSpec.describe 'Creating a Snippet' do
         end.to change { Snippet.count }.by(1)
 
         snippet = Snippet.last
-        created_file_1 = snippet.repository.blob_at('HEAD', file_1[:filePath])
-        created_file_2 = snippet.repository.blob_at('HEAD', file_2[:filePath])
+        created_file_1 = snippet.repository.blob_at(snippet.default_branch, file_1[:filePath])
+        created_file_2 = snippet.repository.blob_at(snippet.default_branch, file_2[:filePath])
 
         expect(created_file_1.data).to match(file_1[:content])
         expect(created_file_2.data).to match(file_2[:content])

spec/requests/api/project_snippets_spec.rb

@@ -138,7 +138,7 @@ RSpec.describe API::ProjectSnippets do
         aggregate_failures do
           expect(snippet.repository.exists?).to be_truthy
 
-          blob = snippet.repository.blob_at('master', file_path)
+          blob = snippet.repository.blob_at(snippet.default_branch, file_path)
 
           expect(blob.data).to eq file_content
         end

spec/requests/api/snippets_spec.rb

@@ -223,7 +223,7 @@ RSpec.describe API::Snippets, factory_default: :keep do
       it 'commit the files to the repository' do
         subject
 
-        blob = snippet.repository.blob_at('master', file_path)
+        blob = snippet.repository.blob_at(snippet.default_branch, file_path)
 
         expect(blob.data).to eq file_content
       end