feature/tpm: use withSRK to probe TPM availability (#17627)

On some platforms e.g. ChromeOS the owner hierarchy might not always be
available to us. To avoid stale sealing exceptions later we probe to
confirm it's working rather than rely solely on family indicator status.

Updates #17622

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

feature/tpm/tpm_test.go

@@ -146,6 +146,18 @@ func BenchmarkInfo(b *testing.B) {
 	b.StopTimer()
 }
 
+func BenchmarkTPMSupported(b *testing.B) {
+	b.StopTimer()
+	skipWithoutTPM(b)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if !tpmSupported() {
+			b.Fatalf("tpmSupported returned false")
+		}
+	}
+	b.StopTimer()
+}
+
 func BenchmarkStore(b *testing.B) {
 	skipWithoutTPM(b)
 	b.StopTimer()