Improve the perl testsuite and add helper functions

[jajik]

While adding more tests in c2d524e I got some ideas for improvements:

• the END block should be added universally to all tests
• the "lib" should provide additional functions
  • universal cleanup (removes all nodes and optionally waits?)
  • helper functions for filtering nodes with OK status (waiting 20+ seconds for a node to get removed is annoying)
  • we should skip if the mod_proxy_cluster module is not configured
  • remove the hardcoded version string shown in mod_manager's html output (we should probably check it with regex – mod_cluster/)