tstest/natlab/vmtest: start migrating old natlab tests to vmtest (#19727)

Instead of having two entry points for running natlab tests, start
converting the connectivity tests to use the vmtest framework.

Grid and pair tests have yet to be moved over.

Updates #13038

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

.github/workflows/natlab-basic.yml

@@ -42,4 +42,4 @@ jobs:
           make -C gokrazy natlab
       - name: Run natlab integration tests
         run: |
-          ./tool/go test -v -run=^TestEasyEasy$ -timeout=3m -count=1 ./tstest/integration/nat  --run-vm-tests
+          ./tool/go test -v -run=^TestEasyEasy$ -timeout=3m -count=1 ./tstest/natlab/vmtest --run-vm-tests