Create a test suite for Evince.
    
    This PR fixes issue #188. It adds a test suite to test basic
    functionality of Evince and brings the following features:
    
    * test scripts for various Evince functions.
    * needles to support the Evince test scripts
    * new template variables `TESTPATH` and `POSTINSTALL_LOAD_ALL` (see
      below)
    * new logic in `main.py` (see below)
    
    The new variables and the new logic make it easier to create test
    suites for post-installation tests. If TESTPATH is used, OpenQA
    will take all tests mentioned in POSTINSTALL from that specified
    TESTPATH. If both TESTPATH and POSTINSTALL_LOAD_ALL are used, then
    OpenQA will run all tests it can find at the TESTPATH location.
    If POSTINSTALL and POSTINSTALL_LOAD_ALL are set simultaneously,
    then only POSTINSTALL will be taken into account and OpenQA will
    only load tests mentioned there.