Make the test more consistent

spec/type-compat.test.sh

@@ -23,14 +23,14 @@ echo "$s|$i|$j"
 
 ### append in arith context
 declare s
-(( s=1 ))
-(( s+=2 ))  # arith add
+(( s='1 '))
+(( s+=' 2 '))  # arith add
 declare -i i
-(( i=1 ))
-(( i+=2 ))
+(( i='1 ' ))
+(( i+=' 2 ' ))
 declare -i j
-(( j=x ))  # treated like zero
-(( j+=2 ))
+(( j='x ' ))  # treated like zero
+(( j+=' 2 ' ))
 echo "$s|$i|$j"
 # stdout: 3|3|2