logictestccl: add gateway region as metamorphic test variable

[rytaft]

Metamorphic testing in logictests supports varying many different settings while ensuring that the output of each SQL query is always the same. We should add an extra metamorphic variable for ccllogictests, which varies the region of the gateway node. This may allow us to uncover bugs like #73024, since INSERT behavior is different depending on the region of insertion.

Jira issue: CRDB-11832

[srosenberg]

@rytaft This is great! Is the idea simply to randomize query.nodeIdx instead of the current behavior which defaults to 0 unless otherwise specified (in testdata)?

[rytaft]

@srosenberg That sounds right to me -- I haven't dug into the specifics of how it currently works, but that sounds like the right approach.

[cucaroach]

Two thoughts: reduce noise/intrusiveness by scoping to ccllogic tests, also maybe limit to setting the metamorphic node id on file basis instead of query by query.

[rytaft]

Moving to backlog since it seems like this is harder than we thought before.