test_file_exhaustive.rb: writable open

* test/ruby/test_file_exhaustive.rb (test_flock_shared): open
  in writable mode for exclusive lock.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55221 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_file_exhaustive.rb

@@ -1290,7 +1290,7 @@ class TestFileExhaustive < Test::Unit::TestCase
       end;
       assert_separately(["-rtimeout", "-", regular_file], "#{<<~begin}#{<<~"end;"}")
       begin
-        open(ARGV[0], "r") do |f|
+        open(ARGV[0], "r+") do |f|
           assert_raise(Timeout::Error) do
             Timeout.timeout(0.1) do
               f.flock(File::LOCK_EX)