Fix invalid dummy secret length for `Rack::Session::Cookie` in tests

[r7kamura]

This addresses an issue where some tests fail when using the recent rack-session gem.

In recent versions of Rack::Session::Cookie, passing a secret shorter than 64 characters raises an ArgumentError. Therefore, I replaced the dummy value used in the tests with a suitable 64-character string.

I ran this library’s test suite on Ruby 3.4 and confirmed that the test cases which had previously been failing now pass as a result of this change.