Clean up RenderEngineTest

Split out from Ib9b8abb25f4894f1b50b6286720c2abf89ca74e2, since they're
orthogonal.

Remove `skip()`, which I believe stopped being called in
I69119623b194885bcc4cf2ddc8e592576b713b19.

Remove SkiaGLESCMRenderEngineFactory, which hasn't been used since
I761cbd3967d1800cc74538f93f1b9c3ffe11ee6a.

Consolidate createRenderEngine (and the trivial
createSkiaVkRenderEngine) into the base class, since all versions were
doing essentially the same thing.

Bug: NA
Test: atest librenderengine_test
Change-Id: Iad446bfa39e371f34db76e2aecfd1fbfb706fc02