Fixes#9098
## Problem
`test_readonly_node_gc` is flaky. As shown in [Allure
Report](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9469/11444519440/index.html#suites/3ccffb1d100105b98aed3dc19b717917/2c02073738fa2b39),
we would get a `AssertionError: No layers should be removed, old layers
are guarded by leases.` after the test restarts pageservers or after
reconfigure pageservers.
During the investigation, we found that the layers has LSN (`0/1563088`)
greater than the LSN (`0x1562000`) protected by the lease. For instance,
**Layers removed**
<pre>
000000067F00000005000034540100000000-000000067F00000005000040050100000000__000000000<b><i>1563088</i></b>-00000001
(shard 0002)
000000068000000000000017E20000000001-010000000100000001000000000000000001__000000000<b><i>1563088</i></b>-00000001
(shard 0002)
</pre>
**Lsn Lease Granted**
<pre>
handle_make_lsn_lease{lsn=<b><i>0/1562000</i></b> shard_id=0002
shard_id=0002}: lease created, valid until 2024-10-21
</pre>
This means that these layers are not guarded by the leases: they are in
"future", not visible to the static endpoint.
## Summary of changes
- Remove the assertion layers_removed == 0 after trigger timeline GC
while holding the lease. Instead rely on the successful execution of
the`SELECT` query to test lease validity.
- Improve test logging
Signed-off-by: Yuchen Liang <yuchen@neon.tech>
db900ae9d0