tstest/natlab, .github/workflows: add opt-in natlab CI workflow

The natlab vmtest suite (tstest/natlab/vmtest) and the integration nat
tests are gated behind --run-vm-tests because they need KVM and are
slow. Until now nothing in CI exercised them apart from a single
canary TestEasyEasy run on every PR.

Add .github/workflows/natlab-test.yml that runs the full opt-in suite
on demand (workflow_dispatch), on PRs labeled "natlab", and on main
every 12 hours via cron. The workflow has two phases:

  - "prepare" builds the gokrazy VM image, downloads the Ubuntu and
    FreeBSD cloud images once via the new natlabprep tool, and emits
    a dynamic JSON matrix of every TestX function it finds in the two
    opt-in packages.
  - "test" is a per-test matrix that depends on prepare. Each matrix
    job restores the shared caches and runs a single test, so adding
    a new TestFoo is automatically picked up on the next run without
    any workflow edits.

Rename the existing natlab-integrationtest.yml to natlab-basic.yml
since it's the small smoke variant (just TestEasyEasy on every PR);
the new natlab-test.yml is the bigger suite. The job inside is
renamed to EasyEasy for the same reason.

Move the macOS arm64 host check from vmtest.Env.Start into
vmtest.Env.AddNode so a test that adds a vmtest.MacOS node skips
immediately on a non-macOS host, and add an explicit
skipIfNotMacOSArm64 helper at the top of the two macOS-only tests
so the platform requirement is obvious to readers.

Quiet the takeAgentConnOne miss log in tstest/natlab/vnet by default
(it was the overwhelming majority of bytes in CI logs, with no signal
in healthy runs) and replace it with a periodic "still waiting" line
that only fires after 10s, so a truly stuck agent connection still
surfaces.

Updates #13038

Change-Id: I4582098d8865200fd5a73a9b696942319ccf3bf0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vmtest/images.go

@@ -91,6 +91,22 @@ var (
 	}
 )
 
+// CloudImages returns the set of QEMU-bootable cloud OS images natlab can
+// use for vmtests, excluding gokrazy (built from source) and macOS (which
+// uses a separate snapshot pipeline). It is intended for tooling such as
+// a CI prep step that wants to warm the image cache.
+func CloudImages() []OSImage {
+	return []OSImage{Ubuntu2404, Debian12, FreeBSD150}
+}
+
+// EnsureImage downloads img to the local cache if not already present.
+// It is intended for tooling that wants to warm the image cache before
+// running natlab vmtests (e.g. a CI prep step). The test framework also
+// calls into the package-internal equivalent on demand.
+func EnsureImage(ctx context.Context, img OSImage) error {
+	return ensureImage(ctx, img)
+}
+
 // imageCacheDir returns the directory for cached VM images.
 func imageCacheDir() string {
 	if d := os.Getenv("VMTEST_CACHE_DIR"); d != "" {