Fix confusion between Postgres and pageserver connection string in test.

We passed the pageserver's libpq endpoint URL as the 'compute_ctl
--connstr' argument, but that was bogus: the --connstr URL is supposed
to be the URL to the *Postgres* instance that compute_ctl launches and
monitors, not to the pageserver. compute_ctl does need the pageserver
URL too, but it is read from the cluster spec JSON, not --connstr.

That was pretty confusing, as you got a lot of "unknown command"
errors in the pageserver log, when compute_tools tries to run regular
SQL commands on the pageserver. The test still passed, however, as it
doesn't require the SQL commands to succeed. But to make this less
confusing, use an invalid hostname instead, so that the queries will
fail to even connect.

test_runner/regress/test_compute_ctl.py

@@ -179,7 +179,16 @@ def test_sync_safekeepers_logs(neon_env_builder: NeonEnvBuilder, pg_bin: PgBin):
     # run compute_ctl and wait for 10s
     try:
         ctl.raw_cli(
-            ["--connstr", ps_connstr, "--pgdata", pgdata, "--spec", spec, "--pgbin", pg_bin_path],
+            [
+                "--connstr",
+                "postgres://invalid/",
+                "--pgdata",
+                pgdata,
+                "--spec",
+                spec,
+                "--pgbin",
+                pg_bin_path,
+            ],
             timeout=10,
         )
     except TimeoutExpired as exc: