Make MGS available in nexus test suite

[rcgoodfellow]

The test_port_settings_basic_crud test is currently disabled because it requires that nexus talk to MGS to figure out which switches it needs to talk to land port settings on a particular switch.

omicron/nexus/tests/integration_tests/switch_port.rs

Lines 28 to 33 in e0a1184

	// TODO: unfortunately this test can no longer be run in the integration test
	// suite because it depends on communicating with MGS which is not part
	// of the infrastructure available in the integration test context.
	#[ignore]
	#[nexus_test]
	async fn test_port_settings_basic_crud(ctx: &ControlPlaneTestContext) {

When this test was originally written, I believe we only had support for one switch and MGS was not involved in helping nexus make decisions. But now that we have multi-switch support, things are more complicated. This may ultimately not be necessary depending on how the following issues pan out e.g., if nexus determines what switches to talk to in ways that do not involve MGS.

• Replace HashMap<SwitchLocation, Client> with on-demand lookups, and eventually DNS #5092
• Switch zone services need per-rack distinction in DNS #5201

[sunshowers]

I think for this and other related issues, we should effectively pin the version of mgd etc. We could either build it within omicron, or have a nextest setup script that can ensure that the right version of mgd (etc) is present on disk (and if not then download it). We should also consider enforcing consistency for non-test environments as well -- again, using either approach.

[davepacheco]

Is it MGD or MGS that we need to be available in the test suite? MGS has been available in the test suite for some time (since at least #5239).

[rcgoodfellow]

This test was pulled in #4390, so we can likely retry this test now with the additional infrastructure from #5239 and #4291.