test: shutdown http test servers (#2559)

## Description
Shutdown HTTP test servers with `Close()`

## Related Issue
Noticed we aren't shutting down HTTP test servers in a couple of our
unit tests while working on #2558

## Checklist before merging

- [x] Test, docs, adr added or updated as needed
- [x] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/.github/CONTRIBUTING.md#developer-workflow)
followed

src/pkg/packager/sources/new_test.go

@@ -130,6 +130,7 @@ func TestPackageSource(t *testing.T) {
 		defer f.Close()
 		io.Copy(rw, f)
 	}))
+	t.Cleanup(func() { ts.Close() })
 
 	tests := []struct {
 		name   string

src/pkg/utils/network_test.go

@@ -85,6 +85,7 @@ func TestDownloadToFile(t *testing.T) {
 		}
 		rw.Write([]byte(content))
 	}))
+	t.Cleanup(func() { srv.Close() })
 
 	tests := []struct {
 		name        string