Setup bootsnap to speedup TestRunnerTest

When running `rails t` we skip the "fork" optimization. Because
of this Rails is loaded from scratch.

However the vast majority of the code is Rails and other gems
so it's the same for all tests.

By seting up bootsnap we can dramatically speed it up.

main:
```
$ bin/test test/application/test_runner_test.rb -n test_run_single_file
Run options: -n test_run_single_file --seed 8478

.

Finished in 7.313216s, 0.1367 runs/s, 0.2735 assertions/s.
```

This branch:
```
$ bin/test test/application/test_runner_test.rb -n test_run_single_file
Run options: -n test_run_single_file --seed 60063

.

Finished in 2.751576s, 0.3634 runs/s, 0.7269 assertions/s.
1 runs, 2 assertions, 0 failures, 0 errors, 0 skips
```

Author: byroot

railties/test/isolation/abstract_unit.rb

@@ -38,6 +38,10 @@ def app_template_path
       File.join RAILS_FRAMEWORK_ROOT, "tmp/templates/app_template"
     end
 
+    def bootsnap_cache_path
+      File.join RAILS_FRAMEWORK_ROOT, "tmp/templates/bootsnap"
+    end
+
     def tmp_path(*args)
       @tmp_path ||= File.realpath(Dir.mktmpdir(nil, File.join(RAILS_FRAMEWORK_ROOT, "tmp")))
       File.join(@tmp_path, *args)
@@ -368,7 +372,12 @@ def rails(*args, allow_failure: false, stderr: false)
         Process.waitpid pid
 
       else
-        output = `cd #{app_path}; #{command}`
+        ENV["BOOTSNAP_CACHE_DIR"] = bootsnap_cache_path
+        begin
+          output = `cd #{app_path}; #{command}`
+        ensure
+          ENV.delete("BOOTSNAP_CACHE_DIR")
+        end
       end
 
       raise "rails command failed (#{$?.exitstatus}): #{command}\n#{output}" unless allow_failure || $?.success?
@@ -513,6 +522,7 @@ def self.sh(cmd)
 
   sh "#{Gem.ruby} #{RAILS_FRAMEWORK_ROOT}/railties/exe/rails new #{app_template_path} --skip-bundle --no-rc --quiet"
   File.open("#{app_template_path}/config/boot.rb", "w") do |f|
+    f.puts 'require "bootsnap/setup" if ENV["BOOTSNAP_CACHE_DIR"]'
     f.puts 'require "rails/all"'
   end