As a result of the patch, all required files are moved to app-luatest
subdirectory. introducing the following layout:

  app-luatest/
  |- <name>_test.lua
  |- lib/
     |- CMakeLists.txt
     |- <libname>.c

The test itself is rewritten via <justrun> and <treegen> helpers to make
it more clear and easy to maintain.

Since luatest provides <skip_if> helper, .skipcond file is not necessary
anymore and is removed.

Needed for #8252

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring