tests: simplify code around unstable test_basebackup_with_high_slru_count (#8477)

## Problem

In `test_basebackup_with_high_slru_count`, the pageserver is sometimes
mysteriously hanging on startup, having been started+stopped earlier in
the test setup while populating template tenant data.

- #7586 

We can't see why this is hanging in this particular test. The test does
some weird stuff though, like attaching a load of broken tenants and
then doing a SIGQUIT kill of a pageserver.

## Summary of changes

- Attach tenants normally instead of doing a failpoint dance to attach
them as broken
- Shut the pageserver down gracefully during init instead of using
immediate mode
- Remove the "sequential" variant of the unstable test, as this is going
away soon anyway
- Log before trying to acquire lock file, so that if it hangs we have a
clearer sense of if that's really where it's hanging. It seems like it
is, but that code does a non-blocking flock so it's surprising.

test_runner/performance/pageserver/pagebench/test_large_slru_basebackup.py

@@ -17,13 +17,11 @@ from performance.pageserver.util import (
 @pytest.mark.parametrize("duration", [30])
 @pytest.mark.parametrize("pgbench_scale", [get_scale_for_db(200)])
 @pytest.mark.parametrize("n_tenants", [10])
-@pytest.mark.parametrize("get_vectored_impl", ["sequential", "vectored"])
 @pytest.mark.timeout(1000)
 def test_basebackup_with_high_slru_count(
     neon_env_builder: NeonEnvBuilder,
     zenbenchmark: NeonBenchmarker,
     pg_bin: PgBin,
-    get_vectored_impl: str,
     n_tenants: int,
     pgbench_scale: int,
     duration: int,
@@ -47,7 +45,7 @@ def test_basebackup_with_high_slru_count(
     max_file_descriptors = 500000
     neon_env_builder.pageserver_config_override = (
         f"page_cache_size={page_cache_size}; max_file_descriptors={max_file_descriptors}; "
-        f"get_vectored_impl='{get_vectored_impl}'; validate_vectored_get=false"
+        f"get_vectored_impl='vectored'; validate_vectored_get=false"
     )
     params.update(
         {