Migrate test framework to Test2

At least Fedora 30 will have no Test::More anymore. Test::More is
deprecated, as well as Test::Stream and Test2 is a recommended
replacement.

Luckily, gost-engine's use of Test::More is limited to functions which
are compatible with implementations in Test2, so only module import and
test plan announcement need to change.

Also, on Fedora 29 perl 5.28 is very picky on undefined variables.