[ruby/irb] Add tests for RubyLex

The set_auto_indent method calculates the correct number of spaces for
indenting a line. We think there might be a few bugs in this method so
we are testing the current functionality to make sure nothing breaks
when we address those bugs.

Example test failure:

```
  1) Failure:
TestIRB::TestRubyLex#test_auto_indent [/Users/Ben/Projects/irb/test/irb/test_ruby_lex.rb:75]:
Calculated the wrong number of spaces for:
 def each_top_level_statement
  initialize_input
  catch(:TERM_INPUT) do
    loop do
      begin
        prompt
        unless l = lex
          throw :TERM_INPUT if @line == ''
        else
.
<10> expected but was
<12>.
```

https://github.com/ruby/irb/commit/752d5597ab

test/irb/test_ruby_lex.rb

@@ -0,0 +1,79 @@
+require 'test/unit'
+
+module TestIRB
+  class TestRubyLex < Test::Unit::TestCase
+    Row = Struct.new(:content, :current_line_spaces, :new_line_spaces)
+
+    class MockIO
+      def initialize(params, &assertion)
+        @params = params
+        @assertion = assertion
+      end
+
+      def auto_indent(&block)
+        result = block.call(*@params)
+        @assertion.call(result)
+      end
+    end
+
+    def assert_indenting(lines, correct_space_count, add_new_line)
+      lines = lines + [""] if add_new_line
+      last_line_index = lines.length - 1
+      byte_pointer = lines.last.length
+
+      ruby_lex = RubyLex.new()
+      io = MockIO.new([lines, last_line_index, byte_pointer, add_new_line]) do |auto_indent|
+        error_message = "Calculated the wrong number of spaces for:\n #{lines.join("\n")}"
+        assert_equal(correct_space_count, auto_indent, error_message)
+      end
+      ruby_lex.set_input(io)
+      context = OpenStruct.new(auto_indent_mode: true)
+      ruby_lex.set_auto_indent(context)
+    end
+
+    def test_auto_indent
+      input_with_correct_indents = [
+        Row.new(%q(def each_top_level_statement), nil, 2),
+        Row.new(%q(  initialize_input), nil, 2),
+        Row.new(%q(  catch(:TERM_INPUT) do), nil, 4),
+        Row.new(%q(    loop do), nil, 6),
+        Row.new(%q(      begin), nil, 8),
+        Row.new(%q(        prompt), nil, 8),
+        Row.new(%q(        unless l = lex), nil, 10),
+        Row.new(%q(          throw :TERM_INPUT if @line == ''), nil, 10),
+        Row.new(%q(        else), 8, 10),
+        Row.new(%q(          @line_no += l.count("\n")), nil, 10),
+        Row.new(%q(          next if l == "\n"), nil, 10),
+        Row.new(%q(          @line.concat l), nil, 10),
+        Row.new(%q(          if @code_block_open or @ltype or @continue or @indent > 0), nil, 12),
+        Row.new(%q(            next), nil, 12),
+        Row.new(%q(          end), 10, 10),
+        Row.new(%q(        end), 8, 8),
+        Row.new(%q(        if @line != "\n"), nil, 10),
+        Row.new(%q(          @line.force_encoding(@io.encoding)), nil, 10),
+        Row.new(%q(          yield @line, @exp_line_no), nil, 10),
+        Row.new(%q(        end), 8, 8),
+        Row.new(%q(        break if @io.eof?), nil, 8),
+        Row.new(%q(        @line = ''), nil, 8),
+        Row.new(%q(        @exp_line_no = @line_no), nil, 8),
+        Row.new(%q(        ), nil, 8),
+        Row.new(%q(        @indent = 0), nil, 8),
+        Row.new(%q(      rescue TerminateLineInput), 6, 8),
+        Row.new(%q(        initialize_input), nil, 8),
+        Row.new(%q(        prompt), nil, 8),
+        Row.new(%q(      end), 6, 6),
+        Row.new(%q(    end), 4, 4),
+        Row.new(%q(  end), 2, 2),
+        Row.new(%q(end), 0, 0),
+      ]
+
+      lines = []
+      input_with_correct_indents.each do |row|
+        lines << row.content
+
+        assert_indenting(lines, row.current_line_spaces, false)
+        assert_indenting(lines, row.new_line_spaces, true)
+      end
+    end
+  end
+end