FIX: `docker:test` Rake task did not run system tests in parallel (#24507)

Why this change?

We support a `USE_TURBO` environment variable which tells the
`docker:test` rake task to run rspec tests in parallel. However, this
currently does not apply to system tests.

What does this change do?

This commit runs system specs for both core and plugins using
`./bin/turbo_rspec` when the `USE_TURBO` environment is present. Note
that when running system specs, we will only spawn X number of test
processes where X is half the number of available CPU cores. This is
done because we have to leave CPU resources for the chrome processes
that will be created.

lib/tasks/docker.rake

@@ -78,6 +78,10 @@ def migrate_databases(parallel: false, load_plugins: false)
   success
 end
 
+def system_tests_parallel_tests_processors_env
+  "PARALLEL_TEST_PROCESSORS=#{Etc.nprocessors / 2}"
+end
+
 # Environment Variables (specific to this rake task)
 desc "Setups up the test environment"
 task "docker:test:setup" do
@@ -214,6 +218,8 @@ task "docker:test" do
         )
 
       unless ENV["JS_ONLY"]
+        @good &&= run_or_fail("bin/ember-cli --build") if ENV["RUN_SYSTEM_TESTS"]
+
         if ENV["WARMUP_TMP_FOLDER"]
           run_or_fail("bundle exec rspec ./spec/requests/groups_controller_spec.rb")
         end
@@ -236,14 +242,21 @@ task "docker:test" do
           end
 
           if ENV["RUN_SYSTEM_TESTS"]
-            @good &&= run_or_fail("bin/ember-cli --build")
-            @good &&= run_or_fail("timeout --verbose 1800 bundle exec rspec spec/system")
+            @good &&=
+              if ENV["USE_TURBO"]
+                run_or_fail(
+                  "#{system_tests_parallel_tests_processors_env} timeout --verbose 1800 bundle exec ./bin/turbo_rspec spec/system",
+                )
+              else
+                run_or_fail("timeout --verbose 1800 bundle exec rspec spec/system")
+              end
           end
         end
 
         unless ENV["SKIP_PLUGINS"]
           if ENV["SINGLE_PLUGIN"]
             @good &&= run_or_fail("bundle exec rake plugin:spec['#{ENV["SINGLE_PLUGIN"]}']")
+
             if ENV["RUN_SYSTEM_TESTS"]
               @good &&=
                 run_or_fail(
@@ -254,11 +267,18 @@ task "docker:test" do
             fail_fast = "RSPEC_FAILFAST=1" unless ENV["SKIP_FAILFAST"]
             task = ENV["USE_TURBO"] ? "plugin:turbo_spec" : "plugin:spec"
             @good &&= run_or_fail("#{fail_fast} bundle exec rake #{task}")
+
             if ENV["RUN_SYSTEM_TESTS"]
               @good &&=
-                run_or_fail(
-                  "LOAD_PLUGINS=1 timeout --verbose 1600 bundle exec rspec plugins/*/spec/system".strip,
-                )
+                if ENV["USE_TURBO"]
+                  run_or_fail(
+                    "LOAD_PLUGINS=1 #{system_tests_parallel_tests_processors_env} timeout --verbose 1600 bundle exec ./bin/turbo_rspec plugins/*/spec/system",
+                  )
+                else
+                  run_or_fail(
+                    "LOAD_PLUGINS=1 timeout --verbose 1600 bundle exec rspec plugins/*/spec/system",
+                  )
+                end
             end
           end
         end