Add bintest for `mrbc`.

bintest/mrbc.rb

+require 'tempfile'
+
+assert('Compiling multiple files without new line in last line. #2361') do
+  a, b, out = Tempfile.new('a.rb'), Tempfile.new('b.rb'), Tempfile.new('out.mrb')
+  a.write('module A; end')
+  a.flush
+  b.write('module B; end')
+  b.flush
+  result = `bin/mrbc -c -o #{out.path} #{a.path} #{b.path} 2>&1`
+  assert_equal "bin/mrbc:#{a.path}:Syntax OK", result.chomp
+  assert_equal 0, $?.exitstatus
+end

tasks/mruby_build.rake

@@ -209,6 +209,7 @@ module MRuby
 
     def run_bintest
       targets = @gems.select { |v| File.directory? "#{v.dir}/bintest" }.map { |v| filename v.dir }
+      targets << filename(".") if File.directory? "./bintest"
       sh "ruby test/bintest.rb #{targets.join ' '}"
     end