Tezlink/Node: test balance service using octez-client

previous:

• !17532 (merged) add support for endpoint with custom root
• !17535 (merged) /tezlink/describe
• !17489 (merged) .../balance

What

Update test of /balance to use octez-client

Why

It's working, better supported and less ad-hoc than curl.

How

We needed !17532 (merged) to add the support of having a custom root (/tezlink) and a !17535 (merged) to add a /tezlink/describe rpc, and then it's just a question of using octez-client on the endpoint of the sequencer.

Manually testing the MR

If the test is still working we're good.