tests: try harder to find open ports for apache, git, and svn
    
    When running multiple builds, we frequently see failures due to port
    conflicts, particularly with httpd tests.  Retry with a different port
    when the test function start_httpd() fails to reduce these spurious
    failures.
    
    We should not need to skip t9115-git-svn-dcommit-funky-renames as a
    result.  Remove it from GIT_SKIP_TESTS.
    
    Similarly, adjust the git-daemon and svnserve start functions.