Merge from widevine repo of http://go/wvgerrit/14218
If GTEST_FILTER was not set, it defaults to "" which matches no
tests. This CL changes it to "*" if it was "".
Also, quotes are added around the environment variable and command so
that it multiple variables can be set.
Change-Id: Icad9a1fbca2d1b436f6e37a2a080a70a384f0a1d