Add missing glMemoryBarrier between SBSO writes and map in fp64 tests
In order for glMapBuffer to see the results from the SSBO writes in the previous draw's shaders, it's necessary to perform a memory barrier: glMemoryBarrier(GL_BUFFER_UPDATE_BARRIER_BIT).
Fixes failures on iris when preferring GPU blits for transfer maps (to avoid uncached VRAM reads on discrete cards).
Closes: mesa#7930 (closed)