SF: Optionally skip slow unittests

TimeStatsTest::canDumpWithInvalidMaxLayers can take many seconds to run
(even a full minute, depending on the target).

Add a new flag "--no-slow" for libsurfaceflinger_unittests which will
cause it to skip slow tests like these, and add a check to that test
which checks for that flag to skip it.

To run use it with atest, you need to use:

    atest libsurfaceflinger_unittest -- --module-arg libsurfaceflinger_unittest:native-test-flag:--no-slow

Bug: None
Test: atest libsurfaceflinger_unittest

Change-Id: I433c1707eb52866fb9f5c61956db338cc5c58723