New requirement: Ensure Linux/aarch64 can run external tests

[sxa]

We do not currently run the external test suites on platforms other than Linux/x64. With our capacity likely to be reduced on that platform as a result of #3292 we should ensure that we have Linux/aarch64 agents able to run the external tests (and possibly the same on other Linux architectures) and then verify that the tests are suitable for running on that platform.

FYA @smlambert since we spoke about this earlier.