selfupdate: move most code into selfupdate package and add missing tests

Signed-off-by: Steffen Vogel <post@steffenvogel.de>

pkg/selfupdate/github_test.go

@@ -0,0 +1,21 @@
+package selfupdate_test
+
+import (
+	"context"
+	"time"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/stv0g/cunicu/pkg/selfupdate"
+)
+
+var _ = Context("github", func() {
+	It("can get latest release", func() {
+		rel, err := selfupdate.GitHubLatestRelease(context.Background())
+		Expect(err).To(Succeed())
+
+		Expect(rel.Version).To(MatchRegexp(`\d+\.\d+\.\d+`))
+		Expect(rel.PublishedAt).To(BeTemporally("<", time.Now()))
+		Expect(len(rel.Assets)).To(BeNumerically(">", 10))
+	})
+})