internet-latency-collector: pin exchange path behaviour with characterisation tests

[thijsvanemmerik]

The internet latency collector measures round-trip time between DoubleZero exchange locations
using RIPE Atlas probes, and nothing records how it behaves today. This adds one test file to
internal/ripeatlas with three tests that pin that behaviour, so a later change to it shows up
as a test failure.

The three facts are connected. The collector sorts location codes alphabetically, makes one
measurement per pair of locations, and aims the ping at the location that sorts first. On export
the two ends are swapped: the location that was pinged is written as the source, and the location
that did the pinging as the target. The swap is only correct because that ordering exists. Change
the ordering without changing the export and every row is stored with its two ends the wrong way
round, and nothing errors.

No production code is touched.

Test: go test ./controlplane/internet-latency-collector/internal/ripeatlas/ -run Baseline