Add test for bin/dev script

lib/generators/react_on_rails/bin/dev

@@ -8,11 +8,12 @@ rescue Errno::ENOENT
 end
 
 def run(process)
-  argv = ARGV.join(" ")
+  args = [*ARGV]
+  args.shift
   puts "Using #{process}"
-  exec "#{process} start -f Procfile.dev", argv
+  exec "#{process} start -f Procfile.dev", args.join(" ")
 rescue Errno::ENOENT
-  puts <<~MSG
+  warn <<~MSG
     ERROR:
     Please ensure `Procfile.dev` exist in your project!
   MSG
@@ -24,7 +25,7 @@ if installed? "overmind"
 elsif installed? "foreman"
   run "foreman"
 else
-  puts <<~MSG
+  warn <<~MSG
     NOTICE:
     For this script to run, you need either 'overmind' or 'foreman' installed on your machine. Please try this script after installing one of them.
   MSG

spec/react_on_rails/dev_spec.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+# To suppress stdout during tests
+RSpec.configure do |config|
+  original_stderr = $stderr
+  original_stdout = $stdout
+  config.before(:all) do
+    $stderr = File.open(File::NULL, "w")
+    $stdout = File.open(File::NULL, "w")
+  end
+  config.after(:all) do
+    $stderr = original_stderr
+    $stdout = original_stdout
+  end
+end
+
+RSpec.describe "bin/dev script" do
+  let(:script_path) { "lib/generators/react_on_rails/bin/dev" }
+
+  it "with Overmind installed, uses Overmind" do
+    allow(IO).to receive(:popen).with("overmind -v").and_return("Some truthy result")
+
+    expect_any_instance_of(Kernel).to receive(:exec).with("overmind start -f Procfile.dev", "")
+
+    load script_path
+  end
+
+  it "without Overmind and with Foreman installed, uses Foreman" do
+    allow(IO).to receive(:popen).with("overmind -v").and_raise(Errno::ENOENT)
+    allow(IO).to receive(:popen).with("foreman -v").and_return("Some truthy result")
+
+    expect_any_instance_of(Kernel).to receive(:exec).with("foreman start -f Procfile.dev", "")
+
+    load script_path
+  end
+
+  it "without Overmind and Foreman installed, exits with error message" do
+    allow(IO).to receive(:popen).with("overmind -v").and_raise(Errno::ENOENT)
+    allow(IO).to receive(:popen).with("foreman -v").and_raise(Errno::ENOENT)
+    allow_any_instance_of(Kernel).to receive(:exit!)
+
+    expected_message = <<~MSG
+      NOTICE:
+      For this script to run, you need either 'overmind' or 'foreman' installed on your machine. Please try this script after installing one of them.
+    MSG
+
+    expect { load script_path }.to output(expected_message).to_stderr_from_any_process
+  end
+
+  it "With Overmind and without Procfile, exits with error message" do
+    allow(IO).to receive(:popen).with("overmind -v").and_return("Some truthy result")
+
+    allow_any_instance_of(Kernel)
+      .to receive(:exec)
+      .with("overmind start -f Procfile.dev", "")
+      .and_raise(Errno::ENOENT)
+    allow_any_instance_of(Kernel).to receive(:exit!)
+
+    expected_message = <<~MSG
+      ERROR:
+      Please ensure `Procfile.dev` exist in your project!
+    MSG
+
+    expect { load script_path }.to output(expected_message).to_stderr_from_any_process
+  end
+end