Copy the forking isolated test runner from railties

All the tests have a substantial chunk of identical setup effort (in
cases/helper.rb); this makes isolated tests run much faster, without any
change to the variety of how we load files.

activerecord/Rakefile

@@ -66,8 +66,28 @@ end
         adapter_short = adapter == "db2" ? adapter : adapter[/^[a-z0-9]+/]
         puts [adapter, adapter_short].inspect
 
+        dash_i = [
+          "test",
+          "lib",
+          "../activesupport/lib",
+          "../activemodel/lib"
+        ].map { |dir| File.expand_path(dir, __dir__) }
+
+        dash_i.reverse_each do |x|
+          $:.unshift(x) unless $:.include?(x)
+        end
+        $-w = true
+
+        require "bundler/setup" unless defined?(Bundler)
+
+        # Every test file loads this first, so doing it post-fork gains
+        # us nothing.
+        require "cases/helper"
+
         failing_files = []
 
+        test_options = ENV["TESTOPTS"].to_s.split(/[\s]+/)
+
         test_files = (Dir["test/cases/**/*_test.rb"].reject {
           |x| x.include?("/adapters/")
         } + Dir["test/cases/adapters/#{adapter_short}/**/*_test.rb"]).sort
@@ -81,8 +101,24 @@ end
 
         test_files.each do |file|
           puts "--- #{file}"
-          success = sh(Gem.ruby, "-w", "-Itest", file)
-          unless success
+          fake_command = Shellwords.join([
+            FileUtils::RUBY,
+            "-w",
+            *dash_i.map { |dir| "-I#{Pathname.new(dir).relative_path_from(Pathname.pwd)}" },
+            file,
+          ])
+          puts fake_command
+
+          # We could run these in parallel, but pretty much all of the
+          # railties tests already run in parallel, so ¯\_(⊙︿⊙)_/¯
+          Process.waitpid fork {
+            ARGV.clear.concat test_options
+            Rake.application = nil
+
+            load file
+          }
+
+          unless $?.success?
             failing_files << file
             puts "^^^ +++"
           end