Improve test coverage reporting / accuracy (#7819)

* improve test coverage reporting / accuracy

* handle absolute coverpaths

* move tests into multiple files

* rename codecov.yml (https://github.com/codecov/support/issues/426)

cmd/platform/version_test.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package main
+
+import (
+	"testing"
+)
+
+func TestVersion(t *testing.T) {
+	checkCommand(t, "version")
+}