One of the reasons why go-gitea#16447 was needed and why go-gitea#16268 was needed in
the first place was because it appears that editing ldap configuration
doesn't get tested.

This PR therefore adds a basic test that will run the edit pipeline.

In doing so it's now clear that go-gitea#16447 and go-gitea#16268 aren't actually
solving go-gitea#16252. It turns out that what actually happens is that is that
the bytes are actually double encoded.

This PR now changes the json unmarshal wrapper to handle this double
encode.

Fix go-gitea#16252

Signed-off-by: Andrew Thornton <art27@cantab.net>