.github, tstest/natlab/vmtest: replace old VM runner job with natlab tests

The "vm" CI job ran a single test (TestRunUbuntu2404 from
tstest/integration/vms) on a privileged self-hosted runner. Its coverage
is nearly all redundant with the modern natlab vmtest suite, which boots
real Ubuntu VMs and already exercises connectivity, kernel TUN, SSH,
Taildrop, ACME, and OS DNS integration on GitHub-hosted runners.

The two things it tested that natlab didn't are added back as natlab
tests so the runner can be decommissioned:

TestUbuntuSystemdUnit runs tailscaled via the stock systemd unit that
Linux packages ship (cmd/tailscaled/tailscaled.service with
tailscaled.defaults as its EnvironmentFile) instead of launching the
binary directly, verifying the unit's directives and its Type=notify
readiness handshake.

TestDNSExtraRecordsSearchDomains verifies that control-plane DNS
ExtraRecords and search domains are resolvable through the guest's OS
resolver (libc to systemd-resolved to quad-100).

Updates #13038

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I8d0dfb8b8153289e7ca78f3af03dfece9497bfe8

.github/workflows/test.yml

@@ -365,30 +365,6 @@ jobs:
       working-directory: src
       run: ./tool/go test $(./tool/go run ./tool/listpkgs --has-root-tests)
 
-  vm:
-    needs: gomod-cache
-    runs-on: ["self-hosted", "linux", "vm"]
-    # VM tests run with some privileges, don't let them run on 3p PRs.
-    if: github.repository == 'tailscale/tailscale'
-    steps:
-    - name: checkout
-      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      with:
-        path: src
-    - name: Restore Go module cache
-      uses: actions/cache/restore@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
-      with:
-        path: gomodcache
-        key: ${{ needs.gomod-cache.outputs.cache-key }}
-        enableCrossOsArchive: true
-    - name: Run VM tests
-      working-directory: src
-      run: ./tool/go test ./tstest/integration/vms -v -no-s3 -run-vm-tests -run=TestRunUbuntu2404
-      env:
-        HOME: "/var/lib/ghrunner/home"
-        TMPDIR: "/tmp"
-        XDG_CACHE_HOME: "/var/lib/ghrunner/cache"
-
   cross: # cross-compile checks, build only.
     needs: gomod-cache
     strategy:
@@ -912,7 +888,6 @@ jobs:
       - test
       - windows
       - macos
-      - vm
       - cross
       - ios
       - wasm
@@ -958,7 +933,6 @@ jobs:
       - test
       - windows
       - macos
-      - vm
       - cross
       - ios
       - wasm
@@ -1008,7 +982,6 @@ jobs:
       - test
       - windows
       - macos
-      - vm
       - wasm
       - fuzz
       - race-root-integration