Add basic edit ldap auth test & actually fix #16252 (#16465) (#16494)

Add basic edit ldap auth test & actually fix #16252 (#16465) (#16494)

Backport #16465

One of the reasons why #16447 was needed and why #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 #16447 and #16268 aren't actually
solving #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 #16252

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>