Adjust the test suite for rack 3

The project dependencies now allow rack 3 to be installed.  This
backwards incompatible version extracted handlers in a new rackup gem.
While we could indicate we prefer to use rack 2 which is still receiving
update, it is more future-proof to update our code to be compatible with
the latest version of the dependency.

This is a development dependency only so will not affect users of the
project.

riemann-tools.gemspec

@@ -43,6 +43,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency 'github_changelog_generator'
   spec.add_development_dependency 'racc'
+  spec.add_development_dependency 'rackup'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rspec'
   spec.add_development_dependency 'rubocop'

spec/riemann/tools/http_check_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require 'openssl'
-require 'rack/handler/webrick'
+require 'rackup/handler/webrick'
 require 'sinatra/base'
 require 'webrick'
 require 'webrick/https'
@@ -100,7 +100,7 @@ def protected!
           Logger: WEBrick::Log.new(File.open(File::NULL, 'w')),
         }
         @server = WEBrick::HTTPServer.new(server_options)
-        @server.mount('/', Rack::Handler::WEBrick, TestWebserver)
+        @server.mount('/', Rackup::Handler::WEBrick, TestWebserver)
         @started = false
         Thread.new { @server.start }
         Timeout.timeout(1) { sleep(0.1) until @started }