test: restore simplecov

It was broken by #3280, but see
simplecov-ruby/simplecov#1032 for discussion
that led to this workaround.

Author: flavorjones

rakelib/test.rake

@@ -121,6 +121,7 @@ end
 def nokogiri_test_task_configuration(t)
   t.libs << "test"
   t.verbose = true if ENV["TESTGLOB"]
+  t.test_prelude = 'require "simplecov_prelude"' if t.respond_to?(:test_prelude)
 end
 
 def nokogiri_test_case_configuration(t)

test/helper.rb

@@ -12,14 +12,6 @@
 # - NOKOGIRI_MEMORY_SUITE: read more in test/test_memory_usage.rb
 #
 
-unless ENV["RUBY_MEMCHECK_RUNNING"] || ENV["NCPU"]
-  require "simplecov"
-  SimpleCov.start do
-    add_filter "/test/"
-    enable_coverage :branch
-  end
-end
-
 $VERBOSE = true
 
 require "fileutils"

test/simplecov_prelude.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+# https://github.com/simplecov-ruby/simplecov/issues/1032
+unless ENV["RUBY_MEMCHECK_RUNNING"] || ENV["NCPU"].to_i > 1
+  require "simplecov"
+  SimpleCov.start do
+    add_filter "/test/"
+    enable_coverage :branch
+  end
+end