EnvManager: Add Stop method and shutdown server using it in tests

Change-Id: I552db6dc1487ed880bce897c7cec85a89a15355d
diff --git a/core/installer/welcome/env_test.go b/core/installer/welcome/env_test.go
index 448f221..cec2458 100644
--- a/core/installer/welcome/env_test.go
+++ b/core/installer/welcome/env_test.go
@@ -386,6 +386,9 @@
 	if len(httpClient.counts) != 6 {
 		t.Fatal(httpClient.counts)
 	}
+	if err := s.Stop(); err != nil {
+		t.Fatal(err)
+	}
 }
 
 func debugFS(bfs billy.Filesystem, t *testing.T, files ...string) {