Notmuch: fix build on darwin

This patch requires this fix to [talloc](https://github.com/NixOS/nixpkgs/pull/4557)

But with talloc and notmuch patched, I am able to build and run notmuch on darwin.

The first patch changes a bunch of hardcoded paths to test scripts whose shebangs are patched; some of these paths did not exist in my build; not sure why.

The second changes `bin/notmuch` from linking to relative `libnotmuch.3.dylib` to linking to an absolute path, otherwise the dylib is not found at runtime.  The command `otool -L .../bin/notmuch` shows the dylib links on darwin.