Add test case for help command

spec/lrama/option_parser_spec.rb

@@ -10,6 +10,40 @@
     end
   end
 
+  describe "help option" do
+    it "print help and exit" do
+      result = Open3.popen3("ruby", exe_path("lrama"), "--help") do |stdin, stdout, stderr, wait_thr|
+        stdout.read
+      end
+      expect(result).to eq(<<~HELP)
+        Lrama is LALR (1) parser generator written by Ruby.
+
+        Usage: lrama [options] FILE
+
+        Tuning the Parser:
+            -S, --skeleton=FILE              specify the skeleton to use
+            -t                               reserved, do nothing
+
+        Output:
+            -h, --header=[FILE]              also produce a header file named FILE
+            -d                               also produce a header file
+            -r, --report=THINGS              also produce details on the automaton
+                --report-file=FILE           also produce details on the automaton output to a file named FILE
+            -o, --output=FILE                leave output to FILE
+                --trace=THINGS               also output trace logs at runtime
+            -v                               reserved, do nothing
+
+        Error Recovery:
+            -e                               enable error recovery
+
+        Other options:
+            -V, --version                    output version information and exit
+                --help                       display this help and exit
+
+      HELP
+    end
+  end
+
   describe "#validate_report" do
     let(:option_parser) { Lrama::OptionParser.new }