test: Add an integration test for TCP apps.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
controller: Support overwriting image entrypoint

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Use []byte rather than string in TestTCPApp

Signed-off-by: Lewis Marshall <lewis@lmars.net>
bootstrap: Wait for the controller route to respond

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Use the built slugrunner image in TestTCPApp

Signed-off-by: Lewis Marshall <lewis@lmars.net>