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.
