Don't start a new process for every test file

This effectively reverts 200cf32e20,
restoring a variant of 5a0e0e7299.

railties/Rakefile

@@ -11,19 +11,43 @@ task test: "test:isolated"
 
 namespace :test do
   task :isolated do
+    dash_i = [
+      "test",
+      "lib",
+      "../activesupport/lib",
+      "../actionpack/lib",
+      "../actionview/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)
+    require "active_support"
+
     dirs = (ENV["TEST_DIR"] || ENV["TEST_DIRS"] || "**").split(",")
     test_files = dirs.map { |dir| "test/#{dir}/*_test.rb" }
     Dir[*test_files].each do |file|
-      next true if file.include?("fixtures")
-      dash_i = [
-        "test",
-        "lib",
-        "#{__dir__}/../activesupport/lib",
-        "#{__dir__}/../actionpack/lib",
-        "#{__dir__}/../actionview/lib",
-        "#{__dir__}/../activemodel/lib"
-      ]
-      ruby "-w", "-I#{dash_i.join ':'}", file
+      next true if file.start_with?("test/fixtures/")
+
+      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; load file }
+
+      unless $?.success?
+        raise "Command failed with status (#{$?.exitstatus}): #{fake_command}"
+      end
     end
   end
 end