[hardening] github workflows - corrupted cache

aka: ensure that 'make test' works as expected

The cache contains a copy './local' which is - under some circumstance -
corrupted.  It is not possible to clear the cache [1] (see the top of the page).

Ensure that 'make test' works as expected [2] even if

- the python interpreter is missing
- the virtualenv exists but pyyaml is missing

To hardening when the workflow cache fails, this patch adds the new target
'travis.test' into the workflow.  This target probes to import a python module
'yaml'.  If this fails the virtualenv will be completely new build.

[1] https://github.com/actions/cache/issues/2#issuecomment-673493515
[2] https://github.com/searx/searx/pull/2517#discussion_r567240235



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>