util/testenv: add new package to hold InTest

Removes duplicated code.

Updates #cleanup

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/deptest/deptest.go

@@ -15,8 +15,8 @@ import (
 )
 
 type DepChecker struct {
-	GOOS    string
-	GOARCH  string
+	GOOS    string            // optional
+	GOARCH  string            // optional
 	BadDeps map[string]string // package => why
 }