Force binary encoding before matching in source_encoding_spec

After the syntax_suggest sync at 23ea206, raw source bytes leak
into SyntaxError output, which breaks the regex match on CI machines
running under LC_ALL=C where ruby_exe returns a US-ASCII string.

https://rubyci.s3.amazonaws.com/debian/ruby-master/log/20260420T003003Z.fail.html.gz
https://rubyci.s3.amazonaws.com/s390x/ruby-master/log/20260419T230004Z.fail.html.gz
https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20260419T233005Z.fail.html.gz

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

spec/ruby/language/source_encoding_spec.rb

@@ -29,7 +29,7 @@
 
       touch(path, "wb") { |f| f.write source }
       begin
-        ruby_exe(path, args: "2>&1", exit_status: 1).should =~ /invalid multibyte char/
+        ruby_exe(path, args: "2>&1", exit_status: 1).b.should =~ /invalid multibyte char/
       ensure
         rm_r path
       end
@@ -51,7 +51,7 @@
 
       touch(path, "wb") { |f| f.write source }
       begin
-        ruby_exe(path, args: "2>&1", exit_status: 1).should =~ /invalid multibyte char/
+        ruby_exe(path, args: "2>&1", exit_status: 1).b.should =~ /invalid multibyte char/
       ensure
         rm_r path
       end