Add `spike cipher` smoke tests to https://github.com/spiffe/spike/blob/main/hack/bare-metal/startup/start.sh

[v0lkan]

As per our former discussion.

We can chat about the details separately.

cc: @parlakisik .

[v0lkan]

For reference here's a smoke check for the demo workload

# Validate expected output
echo "$DEMO_OUTPUT" | grep -q "SPIKE Demo" || \
  { echo "Error: Missing 'SPIKE Demo' in output"; exit 1; }
echo "$DEMO_OUTPUT" | grep -q "Connected to SPIKE Nexus." || \
  { echo "Error: Missing 'Connected to SPIKE Nexus.' in output"; exit 1; }
echo "$DEMO_OUTPUT" | grep -q "Secret found:" || \
  { echo "Error: Missing 'Secret found:' in output"; exit 1; }
echo "$DEMO_OUTPUT" | grep -q "password: SPIKE_Rocks" || \
  { echo "Error: Missing expected password in output"; exit 1; }
echo "$DEMO_OUTPUT" | grep -q "username: SPIKE" || \
  { echo "Error: Missing expected username in output"; exit 1; }

echo "Demo workload verification passed."

Just running some commands and verifying that the output is what we expect.

So in our case it will be encrypting/decrypting files/streams via spike cipher blah and ensuring that the operation passed w/o issues.