Stop gap solution for long output on test cases - #45882

This patch just changes the inspect method on test case instances.
Seeing test instance internals probably isn't helpful when an exception
is raised (for example a NoMethodError).

This isn't as good as #45122, but should fix #45121

I don't think this is great, but should fix the immediate pain. This does make me think we should have an API in Ruby for telling inspect what instance variables we would like it to use.

The implementation of inspect simply loops over all instance variables, but interestingly it doesn't call the instance_variables method to determine what IVs to inspect.

To me, it feels like these two objects should have different inspect output, but they don't:

class Foo
  def initialize
    @foo = 1
    @bar = 1
  end
end

class Bar < Foo
  def instance_variables
    [:@foo]
  end
end

p Foo.new
p Bar.new

Anyone have any thoughts on this PR or the above thing? I'm happy to file a redmine ticket wrt the inspect feature.