test: compression: Put test variables in a struct

test: compression: Put test variables in a struct

At present the test setup is somewhat mixed with the test itself. But if
the test setup fails (which it should not) then the test is actually
invalid. Put all the test buffers and sizes in a struct and separate out
the core code into a function.

This will make it easier to move the code to use the unit test framework.

Signed-off-by: Simon Glass <sjg@chromium.org>