blueprints: fix group handling

When group names are passed on to dnf, they must be prefixed with an
ampersand, or they are treated as a regular package, potentially
causing the build to fail.

Add a testcase to verify this behavior.

This resolves rhbz#1784035.

Signed-off-by: Tom Gundersen <teg@jklm.no>

internal/blueprint/blueprint.go

@@ -42,7 +42,7 @@ func (b *Blueprint) GetPackages() []string {
 		packages = append(packages, pkg.ToNameVersion())
 	}
 	for _, group := range b.Groups {
-		packages = append(packages, group.Name)
+		packages = append(packages, "@"+group.Name)
 	}
 	return packages
 }