Tezt/Rollup node: test rollup node operating with a remote signer

What

This MR adds a Tezt test to ensure the rollup node works (and continues to work) correctly when operating with a remote signer.

Why

I realized we didn't have any such test while this is a feature of the rollup node (arguably not advertized in the doc).

Manually testing the MR

dune exec tezt/tests/main.exe -- --file sc_rollup.ml remote signer -i

Output should look like:

[13:47:39.485] Starting test: Alpha: wasm_2_0_0 - rollup node can sign operations with remote signer
[13:47:42.662] Starting remote signer
[13:47:42.980] Registering keys as remote in client
[13:47:44.089] Check configuration uses remote signer
[13:47:44.089] Create wallet just for baking
[13:47:44.368] Starting rollup node
[13:47:44.821] Progressing until rollup node publishes and cements
[13:47:46.582] Remote signer signed!
[13:47:48.920] LCC updated to 5
[13:47:49.240] [SUCCESS] (1/1) Alpha: wasm_2_0_0 - rollup node can sign operations with remote signer