This test is checking what happens if you try and define a class in a C
extension where that constant is already not a class. It was doing this
by overriding ::Date and then trying to require 'date. The issue with
this is that if we ever add 'date' as a dependency for the test runner,
this test will break because the test runner files get implicitly
required in an `assert_separately` block.

Better use an explicit class for this purpose which can't be accidentally
required elsewhere.

[Feature #16495]

http://rubyci.s3.amazonaws.com/debian10/ruby-master/log/20230510T123003Z.log.html.gz
```
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/objspace/test_objspace.rb:224: warning: assigned but unused variable - c4
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/ruby/test_class.rb:362: warning: assigned but unused variable - e
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/ruby/test_process.rb:2602: warning: assigned but unused variable - parent_pid
```

The following code produces two NameErrors respectively
and they are independent, but the second one can show
`private constant` message because of first NameError.

```ruby
class C
  class PrivateClass; end
  private_constant :PrivateClass
end

begin
  eval('class C::PrivateClass; end')
rescue => e
  p e
end

begin
  Object.const_get 'Foo'
rescue => e
  p e
end

  #<NameError: private constant C::PrivateClass referenced>
  #<NameError: private constant C::Foo referenced>
  #=> should be #<NameError: uninitialized constant Foo>
```

It fails the test-all tests with
`make test-all TESTS='ruby/class ruby/parse --seed=58891 -v`.

The reason is clear miss from https://github.com/ruby/ruby/commit/7387c08373a

Previously, the frozen check happened on `RCLASS_ORIGIN(self)`, which
can return an iclass. The frozen check is supposed to respond to objects
that users can call methods on while iclasses are hidden from users.
Other mutation methods like Module#{define_method,alias_method,public}
don't do this. Check frozen status on the module itself.

Fixes [Bug #19164] and [Bug #19166].

Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>

Implements [Feature #12084]

Returns the object for which the receiver is the singleton class, or
raises TypeError if the receiver is not a singleton class.

The test was taking 10 seconds on my machine and did timeout
on CI once.