Move steno tests to isolated_specs to prevent mock leakage

The Steno::Sink::Syslog singleton was causing sporadic test failures
when steno tests ran in parallel with other CCNG tests. RSpec doubles
stored in the singleton would leak into other tests, causing
"double leaked into another example" errors.

Instead of adding workarounds to spec_helper.rb, isolate the steno
tests by moving them to spec/isolated_specs/steno/. This follows
the existing pattern for tests that affect global state.

Changes:
- Move spec/unit/lib/steno/ to spec/isolated_specs/steno/
- Update steno spec_helper to call reset! and close Syslog after tests
- Remove global syslog reset hook from main spec_helper.rb
- Remove :skip_syslog_reset tags (no longer needed)

The reset! method on Steno::Sink::Syslog remains as a clean API for
test cleanup within the isolated steno test suite.

Author: johha

spec/unit/lib/steno/codec_rfc3339_spec.rb …olated_specs/steno/codec_rfc3339_spec.rbspec/unit/lib/steno/codec_rfc3339_spec.rb renamed to spec/isolated_specs/steno/codec_rfc3339_spec.rb spec/unit/lib/steno/codec_rfc3339_spec.rb renamed to spec/isolated_specs/steno/codec_rfc3339_spec.rb

@@ -4,9 +4,13 @@
 
 Dir[File.expand_path('support/**/*.rb', __dir__)].each { |file| require file }
 
-# Ensure syslog is closed after each steno test to avoid interfering with other tests
+# Ensure steno state is cleaned up after each test to avoid interfering with other tests
 RSpec.configure do |config|
   config.after do
+    # Reset the Syslog singleton to clear any mocks stored in @syslog and @codec
+    Steno::Sink::Syslog.instance.reset! if defined?(Steno::Sink::Syslog)
+
+    # Close the actual syslog connection if open
     Syslog.close if defined?(Syslog) && Syslog.opened?
   end
 end

spec/unit/lib/steno/spec_helper.rb spec/isolated_specs/steno/spec_helper.rbspec/unit/lib/steno/spec_helper.rb renamed to spec/isolated_specs/steno/spec_helper.rb

@@ -4,7 +4,7 @@
 require 'steno'
 
 RSpec.describe Steno::Config do
-  describe '.from_hash', :skip_syslog_reset do
+  describe '.from_hash' do
     before do
       @log_path = 'some_file'
 
@@ -113,7 +113,7 @@
       expect(config.sinks[0]).to eq(mock_sink)
     end
 
-    it "adds a syslog sink if the 'syslog' key is specified", :skip_syslog_reset do
+    it "adds a syslog sink if the 'syslog' key is specified" do
       write_config(@config_path, { 'syslog' => 'test' })
       mock_sink = double('sink')
       expect(mock_sink).to receive(:open).with('test')