test: Add tools to generate test coverage information

This patch adds some tooling and scripts to generate test coverage
information.

Unfortunately, this involves some hacks as Go does not have support for
generating coverage-enabled binaries, or merging coverage reports; but
overall it's not very intrusive.

test/t-03-queue_persistency/addtoqueue.go

@@ -3,6 +3,9 @@
 //
 // Note that chasquid does NOT support this, we do it before starting up the
 // daemon for testing purposes only.
+//
+// +build ignore
+
 package main
 
 import (