libgui_test: fix a couple minor test issues

This change fixes how the MultiTextureConsumerTest.EGLImageTargetWorks checks
for pixels.  It removes the call to eglSwapBuffers so that the test does not
rely on EGL swap-preserve behavior, and it makes the test use checkPixel()
rather than doing glReadPixels itself.

Bug: 8349336
Change-Id: I0f446a0083eebd07af6fd208762878b4e367725a