Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

routers/web/admin/admin_test.go

@@ -16,64 +16,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestShadowPassword(t *testing.T) {
-	kases := []struct {
-		Provider string
-		CfgItem  string
-		Result   string
-	}{
-		{
-			Provider: "redis",
-			CfgItem:  "network=tcp,addr=:6379,password=gitea,db=0,pool_size=100,idle_timeout=180",
-			Result:   "network=tcp,addr=:6379,password=******,db=0,pool_size=100,idle_timeout=180",
-		},
-		{
-			Provider: "mysql",
-			CfgItem:  "root:@tcp(localhost:3306)/gitea?charset=utf8",
-			Result:   "root:******@tcp(localhost:3306)/gitea?charset=utf8",
-		},
-		{
-			Provider: "mysql",
-			CfgItem:  "/gitea?charset=utf8",
-			Result:   "/gitea?charset=utf8",
-		},
-		{
-			Provider: "mysql",
-			CfgItem:  "user:mypassword@/dbname",
-			Result:   "user:******@/dbname",
-		},
-		{
-			Provider: "postgres",
-			CfgItem:  "user=pqgotest dbname=pqgotest sslmode=verify-full",
-			Result:   "user=pqgotest dbname=pqgotest sslmode=verify-full",
-		},
-		{
-			Provider: "postgres",
-			CfgItem:  "user=pqgotest password= dbname=pqgotest sslmode=verify-full",
-			Result:   "user=pqgotest password=****** dbname=pqgotest sslmode=verify-full",
-		},
-		{
-			Provider: "postgres",
-			CfgItem:  "postgres://user:pass@hostname/dbname",
-			Result:   "postgres://user:******@hostname/dbname",
-		},
-		{
-			Provider: "couchbase",
-			CfgItem:  "http://dev-couchbase.example.com:8091/",
-			Result:   "http://dev-couchbase.example.com:8091/",
-		},
-		{
-			Provider: "couchbase",
-			CfgItem:  "http://user:the_password@dev-couchbase.example.com:8091/",
-			Result:   "http://user:******@dev-couchbase.example.com:8091/",
-		},
-	}
-
-	for _, k := range kases {
-		assert.Equal(t, k.Result, shadowPassword(k.Provider, k.CfgItem))
-	}
-}
-
 func TestSelfCheckPost(t *testing.T) {
 	defer test.MockVariableValue(&setting.PublicURLDetection)()
 	defer test.MockVariableValue(&setting.AppURL, "http://config/sub/")()