test/psych/test_scalar_scanner.rb: Prevent a warning

```
[ 4696/20990] TestAst#test_ranges:test/psych/test_scalar_scanner.rb(none):138: warning: ambiguous first argument; put parentheses or a space even after `-' operator
```
http://rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20210909T123007Z.log.html.gz

test/psych/test_scalar_scanner.rb

@@ -135,7 +135,7 @@ module Psych
 
       assert_equal 1, ss.tokenize('1')
       assert_equal 1, ss.tokenize('+1')
-      assert_equal -1, ss.tokenize('-1')
+      assert_equal(-1, ss.tokenize('-1'))
 
       assert_equal 0b010101010, ss.tokenize('0b010101010')
       assert_equal 0b010101010, ss.tokenize('0b0,1_0,1_,0,1_01,0')