Add launcher test utils

[lok52]

Add test utilities for launcher service:

1. Generic TestDbGuard
2. Server start-up utilities

[rimrakhimov]

I suggest to make it two separate modules available under different set of features (test-database and test-server). For some cases (see eth-bytecode-db//tests) we may need to initialize only database without the server, and in that case it may make sense to use only one of the modules.

Also, maybe it would be possible to drop the database when the test fails? In my experience, it may be become a mess sometimes when you have lots of test cases persisting in the database from all kind of tests. I believe it may still be useful in some cases (but is not in most), so if we could set it explicitly on initialization if the database should be dropped (but drop it by default) would be really great. I'm not sure if that is possible, though