MM-56358: Unskip racy test TestUploadLicenseFile (#26009)

This was already fixed before in https://github.com/mattermost/mattermost/pull/24971.
We just enable the test now.

https://mattermost.atlassian.net/browse/MM-56358

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/license_test.go

@@ -53,7 +53,6 @@ func TestGetOldClientLicense(t *testing.T) {
 }
 
 func TestUploadLicenseFile(t *testing.T) {
-	t.Skip("MM-56359")
 	th := Setup(t)
 	defer th.TearDown()
 	client := th.Client