[PATCH v4 06/12] test: add tests for insert

[David Bremner]

It took longer than I thought (of course) but I finally finished looking
at the first 6 patches. 

I already mentioned a minor man page issue in a seperate message.

I took a second pass through 03/12, and I think I would prefer thethe
control flow of insert_message be closer to the standard style in
notmuch of using a return value variable and a single cleanup block at
the end.  Reasonable people can disagree about issues of style, but in
the end consistency of the code base is also important.

d