Move private repository tests + implement env helper function

Signed-off-by: Guillaume de Rouville <guillaume.derouville@gmail.com>

stdlib/dagger/op/tests/fetch-git/fetch-git.cue

@@ -1,22 +0,0 @@
-package op
-
-import (
-	"alpha.dagger.io/os"
-	"alpha.dagger.io/dagger"
-	"alpha.dagger.io/dagger/op"
-)
-
-// Github PAT
-TestPAT: dagger.#Input & {dagger.#Secret}
-
-TestRepo: #up: [op.#FetchGit & {
-	remote:          "https://github.com/dagger/dagger.git"
-	ref:             "main"
-	authTokenSecret: TestPAT
-}]
-
-TestContent: os.#Container & {
-	always:  true
-	command: "ls -l /input/repo | grep 'universe -> stdlib'"
-	mount: "/input/repo": from: TestRepo
-}