Copy test framework from Git

Git uses a simple and yet powerful test framework, written in shell.
The framework is easy to use for both users and developers so I think
it would help if it is used in notmuch as well.

This is a copy of Git's test framework from commit
b6b0afdc30e066788592ca07c9a6c6936c68cc11 in git repository.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>