MM-19022: Convert config/watcher_test.go t.Fatal calls into require calls (#12524)

* Convert config/watcher_test.go t.Fatal calls into require calls

* Moved wasCalled to a public helper function

Now the testutils package has a new function WasCalled
that can be used by anywhere.

utils/testutils/testutils.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"time"
 
 	"github.com/mattermost/mattermost-server/utils/fileutils"
 )
@@ -27,3 +28,15 @@ func ReadTestFile(name string) ([]byte, error) {
 		return data.Bytes(), nil
 	}
 }
+
+// WasCalled reports whether a given callback channel was called
+// within the specified time duration or not.
+func WasCalled(c chan bool, duration time.Duration) bool {
+	wasCalled := false
+	select {
+	case <-c:
+		wasCalled = true
+	case <-time.After(duration):
+	}
+	return wasCalled
+}