test/ruby/test_file_exhaustive.rb: shorten the name of temporary dir

```
  1) Error:
TestFileExhaustive#test_socket_p:
ArgumentError: too long unix socket path (109bytes given but 108bytes max)
    /export/home/users/chkbuild/cb-sunc/tmp/build/20191207T024036Z/ruby/test/ruby/test_file_exhaustive.rb:155:in `initialize'
```

test/ruby/test_file_exhaustive.rb

@@ -18,7 +18,7 @@ def assert_incompatible_encoding
   end
 
   def setup
-    @dir = Dir.mktmpdir("rubytest-file")
+    @dir = Dir.mktmpdir("ruby-test")
     File.chown(-1, Process.gid, @dir)
   end