test/psych/test_coder.rb: Suppress non-parenthesis warnings

http://rubyci.s3.amazonaws.com/debian9/ruby-master/log/20210518T093002Z.log.html.gz
```
/home/chkbuild/chkbuild/tmp/build/20210518T093002Z/ruby/test/psych/test_coder.rb:277: warning: ambiguity between regexp and two divisions: wrap regexp in parentheses or add a space after `/' operator
```

test/psych/test_coder.rb

@@ -274,7 +274,7 @@ module Psych
 
     def test_coder_style_scalar_default
       foo = Psych.dump 'some scalar'
-      assert_match /\A--- some scalar\n(?:\.\.\.\n)?\z/, foo
+      assert_match(/\A--- some scalar\n(?:\.\.\.\n)?\z/, foo)
     end
 
     def test_coder_style_scalar_any
@@ -282,7 +282,7 @@ module Psych
         scalar: 'some scalar',
         style: Psych::Nodes::Scalar::ANY,
         tag: nil
-      assert_match /\A--- some scalar\n(?:\.\.\.\n)?\z/, foo
+      assert_match(/\A--- some scalar\n(?:\.\.\.\n)?\z/, foo)
     end
 
     def test_coder_style_scalar_plain
@@ -290,7 +290,7 @@ module Psych
         scalar: 'some scalar',
         style: Psych::Nodes::Scalar::PLAIN,
         tag: nil
-      assert_match /\A--- some scalar\n(?:\.\.\.\n)?\z/, foo
+      assert_match(/\A--- some scalar\n(?:\.\.\.\n)?\z/, foo)
     end
 
     def test_coder_style_scalar_single_quoted