Add test

Fixes #95

Author: mattn

test/test1.ok

@@ -51,3 +51,5 @@
 (let *= a 3)
 (let /= a 4)
 (let %= a 5)
+(let ..= a 'foo')
+(echo (concat (concat 'foo' 'bar') 'baz'))

test/test1.vim

@@ -56,3 +56,5 @@ let a += 2
 let a *= 3
 let a /= 4
 let a %= 5
+let a ..= 'foo'
+echo ('foo' .. 'bar')..'baz'