Found hanging test vinyl/ddl.test.lua on:

  [159]  inspector:wait_cond(function() return box.space.test.index.pk:count() == box.space.test.index.tk:count() end)
  [159]  ---
  [159]  - true
  [159]  ...
  [159] -box.snapshot()
  [159] ----
  [159] -- ok
  [159] -...

The real issue happend before it when test failed on:

  [091] --- engine/ddl.result   Thu May 14 16:12:09 2020
  [091] +++ engine/ddl.reject   Fri May 15 04:15:07 2020
  [091] @@ -2558,7 +2558,7 @@
  [091]  ...
  [091]  inspector:wait_cond(function() return box.space.test.index.pk:count() == box.space.test.index.sk:count() end)
  [091]  ---
  [091] -- true
  [091] +- false
  [091]  ...

Our tests have structure when different standalone subtests exists
in the test files. To be able to check all of them this hang must
be neutralized to give the next standalone subtest ability to pass.
To avoid of this hang decided to disable box.snapshot check if the
previous check of the current subtest failed.

Needed for #4353