Fix failing spec due to deprecation warning

silence should return yield. Added new method capture_output to better describe capturing the output.

```
 DEPRECATION WARNING: config.active_support.remove_deprecated_time_with_zone_name is deprecated and will be removed in Rails 7.2. (called from block (2 levels) in <top (required)> at /home/runner/work/flipper/flipper/spec/flipper/engine_spec.rb:32)
 ```

spec/flipper/adapters/strict_spec.rb

@@ -29,13 +29,13 @@
 
     context "#get" do
       it "raises an error for unknown feature" do
-        expect(silence { subject.get(feature) }).to match(/Could not find feature "unknown"/)
+        expect(capture_output { subject.get(feature) }).to match(/Could not find feature "unknown"/)
       end
     end
 
     context "#get_multi" do
       it "raises an error for unknown feature" do
-        expect(silence { subject.get_multi([feature]) }).to match(/Could not find feature "unknown"/)
+        expect(capture_output { subject.get_multi([feature]) }).to match(/Could not find feature "unknown"/)
       end
     end
   end

spec/flipper/engine_spec.rb

@@ -29,7 +29,7 @@
 
   let(:config) { application.config.flipper }
 
-  subject { application.initialize! }
+  subject { SpecHelpers.silence { application.initialize! } }
 
   shared_examples 'config.strict' do
     let(:adapter) { Flipper.adapter.adapter }

spec/flipper/middleware/memoizer_spec.rb

@@ -285,7 +285,7 @@
     end
 
     def get(uri, params = {}, env = {}, &block)
-      silence { super(uri, params, env, &block) }
+      capture_output { super(uri, params, env, &block) }
     end
 
     include_examples 'flipper middleware'

spec/support/spec_helpers.rb

@@ -79,11 +79,22 @@ def silence
     original_stdout = $stdout
 
     # Redirect stderr and stdout
-    output = $stderr = $stdout = StringIO.new
+    $stderr = $stdout = StringIO.new
+
+    yield
+  ensure
+    $stderr = original_stderr
+    $stdout = original_stdout
+  end
+
+  def capture_output
+    original_stderr = $stderr
+    original_stdout = $stdout
+
+    output = $stdout = $stderr = StringIO.new
 
     yield
 
-    # Return output
     output.string
   ensure
     $stderr = original_stderr