Fix thresholds for increment_till_fail

The thresholds that define a successful step of the binary search,
were not read from the test file or initialised to the default value
in case of the increment_till_fail test.

Signed-off-by: Luc Provoost <luc.provoost@gmail.com>
Change-Id: I8347a683f35f8fd2ff26161709bf0bd16387f2e0