Add NotEquals node in parser to simplify expressions so that <value1> != <value2> is
no longer translated internally to NOT(<value1> = <value2>)
Closes: #35210Fixes: #35233
Moves `x-pack/qa/sql/*` into `x-pack/plugin/sql/qa` to make it simpler
to run all of the sql tests. This lines up with how newer projects like
cross cluster replication are testing themselves.