Give a warning or error when unable to remove test/var directory

[Totktonada]

An attempt to proceed with dirty directory may lead to strange effects. Say, reproduce files will contain tests from previous runs. In fact test-run expects that the directory is successfully deleted.

Let's give a warning when shutil.rmtree(args.vardir) in lib/__init__.py is unable to remove the directory. Check that the directory exists first.

Maybe it even worth to give an error, because test-run may work incorrectly in the case.

[Totktonada]

Simple step sequence that may lead to this is when one run testing from root user and then try to do so from a regular user.

[avtikhon]

Seems the only right way is to give an error on such situation, because otherwise some tests may fail as it happened in
tarantool/tarantool-qa#124