runner/settings: set PRUNE_KEEP_ALL as default

In testing drm-tip it is helpful to have reported all tests
results, especially when one have many dynamic ones we want
to get a parent result.

v2: fix for runner_tests (Jari)

Cc: Ewelina Musial <ewelina.musial@intel.com>
Cc: Helen Koike <helen.koike@collabora.com>
Cc: Jari Tahvanainen <jari.tahvanainen@intel.com>
Cc: Petri Latvala <adrinael@adrinael.net>
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Peter Senna Tschudin <peter.senna@intel.com>
Tested-by: Jari Tahvanainen <jari.tahvanainen@intel.com>