If no --variant / --benchmark is specified, it's more reasonable to
prune or run *all* variants/benchmarks (using the wildcard "%")
instead of defaulting to "none"/"none". The trivial case with only
one single variant/benchmark (which may still be "none"/"none" if
import-trace's default is used) is still covered by this new default
behavior.
Change-Id: I0e9001137d5e052183dd74211e2edbcfab749528
0e1ed1feab