Refactor Rainbow usage in specs

We don't test any specific string generated by it yet, so there is no
point in keeping it enabled when we are disabling it everywhere we test its output.

spec/lib/gitlab/bare_repository_import/importer_spec.rb

@@ -10,9 +10,6 @@ describe Gitlab::BareRepositoryImport::Importer, :seed_helper do
   subject(:importer) { described_class.new(admin, bare_repository) }
 
   before do
-    @rainbow = Rainbow.enabled
-    Rainbow.enabled = false
-
     allow(described_class).to receive(:log)
   end
 
@@ -20,7 +17,6 @@ describe Gitlab::BareRepositoryImport::Importer, :seed_helper do
     FileUtils.rm_rf(base_dir)
     TestEnv.clean_test_path
     ensure_seeds
-    Rainbow.enabled = @rainbow
   end
 
   shared_examples 'importing a repository' do

spec/lib/system_check/simple_executor_spec.rb

@@ -98,15 +98,6 @@ describe SystemCheck::SimpleExecutor do
     end
   end
 
-  before do
-    @rainbow = Rainbow.enabled
-    Rainbow.enabled = false
-  end
-
-  after do
-    Rainbow.enabled = @rainbow
-  end
-
   describe '#component' do
     it 'returns stored component name' do
       expect(subject.component).to eq('Test')

spec/spec_helper.rb

@@ -29,6 +29,7 @@ end
 
 # require rainbow gem String monkeypatch, so we can test SystemChecks
 require 'rainbow/ext/string'
+Rainbow.enabled = false
 
 # Requires supporting ruby files with custom matchers and macros, etc,
 # in spec/support/ and its subdirectories.