Add coverage checks

This commit adds a makefile target for generating a report for unit test
coverage. It also adds the coverage_check tool to assert that the actual
test coverage meets the specified requirement.

This also improves various aspects of the testing utilities:
* The "test" package has been renamed to "testutil"
* The "Objs" member has been removed from the CmdTest object
* The "Cmd" member has been added to the CmdTest object. This allows
  testing of multiple variants of a root airshipctl command

Finally, this commit includes additional tests for root.go. These are
required in order to meet the required coverage threshold.

Change-Id: Id48343166c0488c543a405ec3143e4a75355ba43

cmd/testdata/TestRootGoldenOutput/rootCmd-with-defaults.golden

@@ -0,0 +1,20 @@
+airshipctl is a unified entrypoint to various airship components
+
+Usage:
+  airshipctl [command]
+
+Available Commands:
+  argo        argo is the command line interface to Argo
+  bootstrap   bootstraps airshipctl
+  completion  Generate autocompletions script for the specified shell (bash or zsh)
+  help        Help about any command
+  kubeadm     kubeadm: easily bootstrap a secure Kubernetes cluster
+  kubectl     kubectl controls the Kubernetes cluster manager
+  kustomize   Build a kustomization target from a directory or a remote url.
+  version     Show the version number of airshipctl
+
+Flags:
+      --debug   enable verbose output
+  -h, --help    help for airshipctl
+
+Use "airshipctl [command] --help" for more information about a command.

cmd/testdata/TestRootGoldenOutput/specialized-rootCmd-with-bootstrap.golden

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+airshipctl is a unified entrypoint to various airship components
+
+Usage:
+  airshipctl [command]
+
+Available Commands:
+  bootstrap   bootstraps airshipctl
+  help        Help about any command
+  version     Show the version number of airshipctl
+
+Flags:
+      --debug   enable verbose output
+  -h, --help    help for airshipctl
+
+Use "airshipctl [command] --help" for more information about a command.