PERF: Update docker:test to run QUnit with more cores if available (#27816)

This commit updates the `docker:test` rake task to run core and plugin
QUnit tests in parallel using half the number of available CPU
processors to speed up time it takes to run the tests on hardware with
more CPU cores.

Before this commit, core QUnit tests ran by the `docker:test` rake task was capped at 3 parallel
processes while plugin QUnit tests was not ran in parallel.

lib/tasks/docker.rake

@@ -78,8 +78,12 @@ def migrate_databases(parallel: false, load_plugins: false)
   success
 end
 
+def number_of_processors
+  Etc.nprocessors
+end
+
 def system_tests_parallel_tests_processors_env
-  "PARALLEL_TEST_PROCESSORS=#{Etc.nprocessors / 2}"
+  "PARALLEL_TEST_PROCESSORS=#{number_of_processors / 2}"
 end
 
 # Environment Variables (specific to this rake task)
@@ -294,7 +298,7 @@ task "docker:test" do
         unless ENV["SKIP_CORE"]
           @good &&=
             run_or_fail(
-              "cd app/assets/javascripts/discourse && CI=1 yarn ember exam --load-balance --parallel=3 --random",
+              "cd app/assets/javascripts/discourse && CI=1 yarn ember exam --load-balance --parallel=#{number_of_processors / 2} --random",
             )
         end
 
@@ -305,7 +309,10 @@ task "docker:test" do
                 "CI=1 bundle exec rake plugin:qunit['#{ENV["SINGLE_PLUGIN"]}','#{js_timeout}']",
               )
           else
-            @good &&= run_or_fail("CI=1 bundle exec rake plugin:qunit['*','#{js_timeout}']")
+            @good &&=
+              run_or_fail(
+                "QUNIT_PARALLEL=#{number_of_processors / 2}  CI=1 bundle exec rake plugin:qunit['*','#{js_timeout}']",
+              )
           end
         end
       end