tests/misc: Add a test for a load that hits two preceding stores

tests/misc: Add a test for a load that hits two preceding stores

This checks that the store forwarding machinery in the dcache
correctly combines forwarded stores when they are partial stores
(i.e. only writing part of the doubleword, as for a byte store).

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>