test: errinj* tests need fixes for stress runs

Tests failed with the following issues:

[010] --- vinyl/errinj.result Tue Mar 19 17:52:48 2019
[010] +++ vinyl/errinj.reject Tue Mar 19 19:07:58 2019
[010] @@ -81,7 +81,7 @@
[010] -- fails due to scheduler timeout
[010] box.snapshot();
[010] ---
[010] -- error: Error injection 'vinyl dump'
[010] +- ok
[010] ...
[010] fiber.sleep(0.06);
[010] ---
[010]

Decided to remove the current check for box snapshot fail on
scheduler timeout.

[035] --- vinyl/errinj_stat.result	Mon May  6 13:11:27 2019
[035] +++ vinyl/errinj_stat.reject	Mon May  6 17:58:48 2019
[035] @@ -250,7 +250,7 @@
[035]  ...
[035]  box.snapshot()
[035]  ---
[035] -- ok
[035] +- error: Error injection 'vinyl dump'
[035]  ...
[035]  i:compact()
[035]  ---
[035]

Decided to add the wait condition checker of the scheduler
tasks_completed number instead of the fiber delay.

Close #4058

(cherry picked from commit d64e4c95f1b72d1f32f9689a33319545506e1c58)