add test for slice assign

Author: ynkdir

test/test1.ok

@@ -22,6 +22,7 @@
 (let = (dot (dot foo bar) baz) 123)
 (let = (subscript (subscript foo (bar)) (baz)) 456)
 (let = (dot (subscript foo (bar)) baz) 789)
+(let = (slice foo 1 2) (list 3 4))
 (unlet a b c)
 (lockvar a b c)
 (lockvar 1 a b c)

test/test1.vim

@@ -26,6 +26,7 @@ let [a, b; c] .= [1, 2, 3]
 let foo.bar.baz = 123
 let foo[bar()][baz()] = 456
 let foo[bar()].baz = 789
+let foo[1:2] = [3, 4]
 unlet a b c
 lockvar a b c
 lockvar 1 a b c