prober: fix race condition in TestExcludeInRunAll (#18807)

The test was making HTTP requests before waiting for probes to
complete their initial run in "once" mode. This created a race where
sometimes the probe's previous state was empty (0 results) and
sometimes it had one result, causing inconsistent RecentResults and
PreviousSuccessRatio values.

Fixed by waiting for all probes to complete via their stopped channels
before making HTTP requests, matching the pattern used in other tests
like TestProberRunHandler and TestRunAllHandler.

Fixes #18806

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

prober/prober_test.go

@@ -793,9 +793,14 @@ func TestExcludeInRunAll(t *testing.T) {
 		},
 	}
 
-	p.Run("includedProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
-	p.Run("excludedProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
-	p.Run("excludedOtherProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
+	includedProbe := p.Run("includedProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
+	excludedProbe := p.Run("excludedProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
+	excludedOtherProbe := p.Run("excludedOtherProbe", probeInterval, nil, FuncProbe(func(context.Context) error { return nil }))
+
+	// Wait for all probes to complete their initial run
+	<-includedProbe.stopped
+	<-excludedProbe.stopped
+	<-excludedOtherProbe.stopped
 
 	mux := http.NewServeMux()
 	server := httptest.NewServer(mux)