MM-30988 - Fix racy test ServerSystemdNotification (#16431)

* MM-30988 - Fix racy test ServerSystemdNotification

The translateFunc is a global variable which was unguarded.
So we convert that into an atomic variable to prevent against races.

https://mattermost.atlassian.net/browse/MM-30988

```release-note
NONE
```

* fix tests

* move call upwards

* use race

* after translations init

* guard with a sync.Once

* missed import

* revert race

* update timeout

model/utils.go

@@ -18,6 +18,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 	"unicode"
 
@@ -72,10 +73,13 @@ func (sa StringArray) Equals(input StringArray) bool {
 	return true
 }
 
-var translateFunc goi18n.TranslateFunc = nil
+var translateFunc goi18n.TranslateFunc
+var translateFuncOnce sync.Once
 
 func AppErrorInit(t goi18n.TranslateFunc) {
-	translateFunc = t
+	translateFuncOnce.Do(func() {
+		translateFunc = t
+	})
 }
 
 type AppError struct {