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.

cmd/chasquid-util/chasquid-util.go

@@ -1,4 +1,8 @@
 // chasquid-util is a command-line utility for chasquid-related operations.
+//
+// Don't include it in the coverage build.
+// +build !coverage
+
 package main
 
 import (

cmd/dovecot-auth-cli/dovecot-auth-cli.go

@@ -1,6 +1,8 @@
 // CLI used for testing the dovecot authentication package.
 //
 // NOT for production use.
+// +build !coverage
+
 package main
 
 import (

cmd/mda-lmtp/mda-lmtp.go

@@ -1,6 +1,9 @@
 // mda-lmtp is a very basic MDA that uses LMTP to do the delivery.
 //
 // See the usage below for details.
+//
+// +build !coverage
+
 package main
 
 import (

cmd/smtp-check/smtp-check.go

@@ -1,4 +1,7 @@
 // smtp-check is a command-line too for checking SMTP setups.
+//
+// +build !coverage
+
 package main
 
 import (

cmd/spf-check/spf-check.go

@@ -1,6 +1,8 @@
 // Command line tool for playing with the SPF library.
 //
 // Not for use in production, just development and experimentation.
+// +build !coverage
+
 package main
 
 import (