[PORT] Refactor tests to prevent from unnecessary preparations (gitea#32398)

Some preparations are only used by a few tests, so to make the tests fast, they should only be prepared when they are used.

By the way, this PR splits PrepareTestEnv into small functions to make it simple.

---

Conflict resolution: Mostly magical and just re-pasting the code into
the right places.
Done differently: use `require.NoError` instead of `assert.NoError`.

(cherry picked from commit ec2d1593c269e06655525deb96f74b8094221b6f)

modules/util/util.go

@@ -234,6 +234,25 @@ func IfZero[T comparable](v, def T) T {
 	return v
 }
 
+// OptionalArg helps the "optional argument" in Golang:
+//
+//	func foo(optArg ...int) { return OptionalArg(optArg) }
+//		calling `foo()` gets zero value 0, calling `foo(100)` gets 100
+//	func bar(optArg ...int) { return OptionalArg(optArg, 42) }
+//		calling `bar()` gets default value 42, calling `bar(100)` gets 100
+//
+// Passing more than 1 item to `optArg` or `defaultValue` is undefined behavior.
+// At the moment only the first item is used.
+func OptionalArg[T any](optArg []T, defaultValue ...T) (ret T) {
+	if len(optArg) >= 1 {
+		return optArg[0]
+	}
+	if len(defaultValue) >= 1 {
+		return defaultValue[0]
+	}
+	return ret
+}
+
 func ReserveLineBreakForTextarea(input string) string {
 	// Since the content is from a form which is a textarea, the line endings are \r\n.
 	// It's a standard behavior of HTML.