tests: sprintf: increase stack size

A recent patch pinned the stack size for this test at 1024
instead of the platform default. This value wasn't sufficient
on Xtensa. Set to 2048, which was the default on most platforms.

Change-Id: I9a9d5fd448d2377aaf782c2c093a16147f31886a
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>