block: add test bio size define to test-iosched

Add a define for the test bio size (which is the size of a page),
this is used for allocating the right sized buffer for the bio during
test request creation.

Change-Id: I9505c85c4352009bdee442172eb8ae8f4254cfb0
Signed-off-by: Lee Susman <lsusman@codeaurora.org>