test: Adds support to handle different content in log depending on the version

Some functionalities are just enabled on Apache, not part of nginx or IIS. As
example we have the "proxy", currently just supported in Apache. This patches
add to our regression test suite the ability to expect different contents
based on the targert platform.
nginx: Warn about not workable 'proxy'

Proxy is not yet ready for nginx. Instead of give a generic error, this patch
adds a clean message explains that such functionality is not available on the
nginx port. This patch also modifies the test cases to reflect this changes.