Fixed #1649 - incremented interval for tested task to 20ms

[IzabellaRaulin]

Fixes #1649

Summary of changes:

• incrementing interval used in tests from 10ms to 20ms

Reasoning:

• I observed that on Travis max relative error of firing task in interval equals +10.2541 ms (in observed samples), so interval =10 ms caused that sometimes received misses_count were greater than expected).

Solution:

• increase used the interval to 20ms, so tests still will be executed quickly, but the probability of having a greater delay than determined interval has been minimized,

Testing done:

• medium tests

@intelsdi-x/snap-maintainers

[andrzej-k]

LGTM