test696: decouple from test556 data

test696: decouple from test556 data

Test 696 and 556 share the same libtest code. Make sure to issue
the `GET` request to the correct runtime test number instead of using
the hard-wired "556".

It makes the `sws` test server read the response string from `test696`
`<data>` section, instead of reading it from `test556`. To avoid this
hidden interaction between test data.

AFAICS there is no other similar hard-coded string in reused libtests.

Ref: https://github.com/curl/curl/pull/19313#issuecomment-3477448933
Follow-up to be82a3605a4b539580b3de776ffcca25b8770e43 #16003
Closes #19329