Check that the ruby configuration isn't changed during the benchmark

This produces an error like
.../yjit-bench/harness/harness.rb:70:in 'Object#run_benchmark': Benchmark altered ruby configuration! (RuntimeError)
 from ["ruby 3.4.0dev (2024-09-09T14:55:18Z master 1205f17125) [arm64-darwin23]", false]
   to ["ruby 3.4.0dev (2024-09-09T14:55:18Z master 1205f17125) +YJIT [arm64-darwin23]", true]

harness/harness.rb

@@ -16,6 +16,19 @@
 
 system('mkdir', '-p', File.dirname(OUT_CSV_PATH))
 
+def ruby_configuration
+  [
+    # RUBY_DESCRIPTION can change based on enabled features
+    # (and the yjit-metrics reporting will abort if this changes).
+    # Interpolate to separate this string from the original object.
+    "#{RUBY_DESCRIPTION}",
+    # Explicitly check for YJIT toggling as we specifically don't want that.
+    defined?(RubyVM::YJIT) ? RubyVM::YJIT.enabled? : nil,
+  ]
+end
+
+$original_ruby_configuration = ruby_configuration
+
 puts RUBY_DESCRIPTION
 
 def realtime
@@ -52,4 +65,8 @@ def run_benchmark(_num_itrs_hint, &block)
     non_warmups_ms = ((non_warmups.sum / non_warmups.size) * 1000.0).to_i
     puts "Average of last #{non_warmups.size}, non-warmup iters: #{non_warmups_ms}ms"
   end
+
+  if ruby_configuration != $original_ruby_configuration
+    raise "Benchmark altered ruby configuration!\n from #{$original_ruby_configuration.inspect}\n   to #{ruby_configuration.inspect}"
+  end
 end