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@ebea586fdf

docs/en/sql/index.asciidoc

@@ -2,7 +2,7 @@
 [[xpack-sql]]
 = SQL Access
 
-:sql-tests: {docdir}/../../qa/sql
+:sql-tests: {xes-repo-dir}/../../qa/sql
 :sql-specs: {sql-tests}/src/main/resources
 :jdbc-tests: {sql-tests}/src/main/java/org/elasticsearch/xpack/qa/sql/jdbc
 :security-tests: {sql-tests}/security/src/test/java/org/elasticsearch/xpack/qa/sql/security