Allow setting number of parallel workers to 1

In the case that one single test file can't run with more than 1
parallel workers, but the base class has parallelization enabled, we
should still allow the user to set the number of workers to 1.

activesupport/lib/active_support/test_case.rb

@@ -81,8 +81,6 @@ def parallelize(workers: :number_of_processors, with: :processes, threshold: Act
         workers = Concurrent.physical_processor_count if workers == :number_of_processors
         workers = ENV["PARALLEL_WORKERS"].to_i if ENV["PARALLEL_WORKERS"]
 
-        return if workers <= 1
-
         Minitest.parallel_executor = ActiveSupport::Testing::ParallelizeExecutor.new(size: workers, with: with, threshold: threshold)
       end
 

activesupport/lib/active_support/testing/parallelize_executor.rb

@@ -53,7 +53,11 @@ def parallelized?
         end
 
         def should_parallelize?
-          ENV["PARALLEL_WORKERS"] || tests_count > threshold
+          (ENV["PARALLEL_WORKERS"] || tests_count > threshold) && many_workers?
+        end
+
+        def many_workers?
+          size > 1
         end
 
         def tests_count
@@ -67,6 +71,8 @@ def show_execution_info
         def execution_info
           if parallelized?
             "Running #{tests_count} tests in parallel using #{parallel_executor.size} #{parallelize_with}"
+          elsif !many_workers?
+            "Running #{tests_count} tests in a single process"
           else
             "Running #{tests_count} tests in a single process (parallelization threshold is #{threshold})"
           end