test: make connection usable by itself

Frequently when we want to execute commands from different users we need
to pass username and password separatly to each invokation. With this
patch this approach can be simplified and connection can be used
directlry in form of context manager.

Additionally this patch fixes handling of NOLOG env var. When it is
supplied logs are disabled which simplifies debugging of a test nbecause
with continuous output from multiple instances it is impossible to use
debugger efficiently.