Read pixels in BGRA non-premul mode in few tests

Read pixels in BGRA non-premul mode in few tests

The tests assert on Sk_ColorGREEN, which is in BRGA non-premul. Read the
pixels in same color format.

Currently the tests pass on all platforms because GREEN is fully opaque
and the component stays in the same place both on BGRA and
RGBA. However, hypothetically somebody could copy-paste the assertion
for further tests but use, say, RED. This creates latent error that is
only visible on some platforms like mac.

Review URL: https://codereview.chromium.org/989463002