test/uint: fiber

When optimization level increased we've found that
memset calls might be optimized, moreover there
were no check if calls themselves would override
the stack.

Lets rework and use number of calls instead. Strictly
speaking the test is still a bit fragile and precise
testing would rather require asm level code.

Repored-by: "Alexander V. Tikhonov" <avtikhon@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>