SQL: Fix doc pointer to SQL's tests
SQL: Fix doc pointer to SQL's tests

The docs include portions of the SQL tests and for that to work they
need to point to position of the tests. They use a relative directory
but relative to *what*? That turns out to be a fairly complex thing to
answer, luckilly, `index.x.asciidoc` defines `xes-repo-dir` which points
to the root of the xpack docs. We can use that to find the sql tests
without having to answer the "relative to what?" question in two places.

Original commit: elastic/x-pack-elasticsearch@322b7f17221815a30c6e898466e088e7228f9c64

strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra