add support for testing peer connectivity

The new argument `ping` allows users to specify a comma-seperated list
of hosts (IP or hostname) to ping in order to verify connectivity.
Ping is considered successful as soon as the peer is reachable either
directly or via DERP.

Updates tailscale/corp#32817

Signed-off-by: Percy Wegmann <percy@tailscale.com>

.github/workflows/test.yml

@@ -21,6 +21,7 @@ jobs:
             runner-os: Linux
             arch: amd64
             version: latest
+            ping: 100.99.0.2,lax-pve.pineapplefish.ts.net,lax-pve
 
           # Try unstable too
           - os: ubuntu-latest
@@ -45,6 +46,7 @@ jobs:
             runner-os: Windows
             arch: amd64
             version: latest
+            ping: 100.99.0.2,lax-pve.pineapplefish.ts.net,lax-pve
 
           - os: windows-latest
             runner-os: Windows
@@ -62,6 +64,7 @@ jobs:
             runner-os: macOS
             arch: amd64
             version: latest
+            ping: 100.99.0.2 # hostnames aren't resolving on MacOS, just ping IP lax-pve.pineapplefish.ts.net,lax-pve
           
           # macOS ARM
           - os: macos-14
@@ -99,6 +102,7 @@ jobs:
           use-cache: false
           timeout: "5m"
           retry: 3
+          ping: "${{ matrix.ping }}"
       
       # Test Tailscale status command
       - name: Check Tailscale Status