Fix tests and rename request variable in specs to perform_request

Explanation: Using the variable name `request` might interfere with methods in Rails core system prevent successful test execution. See this issue: https://github.com/turbolinks/turbolinks-rails/issues/38

spec/models/code_ocean/error_spec.rb

@@ -0,0 +1,19 @@
+require 'rails_helper'
+
+describe CodeOcean::Error do
+  let(:error) { described_class.create }
+
+  it 'validates the presence of an execution environment' do
+    expect(error.errors[:execution_environment_id]).to be_present
+  end
+
+  it 'validates the presence of a message' do
+    expect(error.errors[:message]).to be_present
+  end
+
+  describe '.nested_resource?' do
+    it 'is true' do
+      expect(described_class.nested_resource?).to be true
+    end
+  end
+end