leak_stack_on_error in test environment (#1557)

Fixes #1554

lib/puma/runner.rb

@@ -22,6 +22,10 @@ module Puma
       @options[:environment] == "development"
     end
 
+    def test?
+      @options[:environment] == "test"
+    end
+
     def log(str)
       @events.log str
     end
@@ -168,7 +172,7 @@ module Puma
         server.early_hints = true
       end
 
-      unless development?
+      unless development? || test?
         server.leak_stack_on_error = false
       end