test_gc_cutoff.py fixes (#2655)

* Fix bogus early exit from GC.

Commit 91411c415a added this failpoint, but the early exit was not
intentional.

* Cleanup test_gc_cutoff.py test.

- Remove the 'scale' parameter, this isn't a benchmark
- Tweak pgbench and pageserver options to create garbage faster that the
  the GC can collect away. The test used to take just under 5 minutes,
  which was uncomfortably close to the default 5 minute test timeout, and
  annoyingly even without the hard limit. These changes bring it down to
  about 1-2 minutes.
- Improve comments, fix typos
- Rename the failpoint. The old name, 'gc-before-save-metadata' implied
  that the failpoint was before the metadata update, but it was in fact
  much later in the function.
- Move the call to persist the metadata outside the lock, to avoid
  holding it for too long.

To verify that this test still covers the original bug,
https://github.com/neondatabase/neon/issues/2539, I commenting out
updating the metadata file like this:
```
diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index 1e857a9a..f8a9f34a 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -1962,7 +1962,7 @@ impl Timeline {
         }
         // Persist the new GC cutoff value in the metadata file, before
         // we actually remove anything.
-        self.update_metadata_file(self.disk_consistent_lsn.load(), HashMap::new())?;
+        //self.update_metadata_file(self.disk_consistent_lsn.load(), HashMap::new())?;

         info!("GC starting");

```
It doesn't fail every time with that, but it did fail after about 5
runs.

pageserver/src/tenant/timeline.rs

@@ -1984,10 +1984,10 @@ impl Timeline {
                 new_gc_cutoff
             );
             write_guard.store_and_unlock(new_gc_cutoff).wait();
-
-            // Persist metadata file
-            self.update_metadata_file(self.disk_consistent_lsn.load(), HashMap::new())?;
         }
+        // Persist the new GC cutoff value in the metadata file, before
+        // we actually remove anything.
+        self.update_metadata_file(self.disk_consistent_lsn.load(), HashMap::new())?;
 
         info!("GC starting");
 
@@ -2114,15 +2114,12 @@ impl Timeline {
         }
 
         info!(
-            "GC completed removing {} layers, cuttof {}",
+            "GC completed removing {} layers, cutoff {}",
             result.layers_removed, new_gc_cutoff
         );
+
         if result.layers_removed != 0 {
-            fail_point!("gc-before-save-metadata", |_| {
-                info!("Abnormaly terinate pageserver at gc-before-save-metadata fail point");
-                std::process::abort();
-            });
-            return Ok(result);
+            fail_point!("after-timeline-gc-removed-layers");
         }
 
         if self.upload_layers.load(atomic::Ordering::Relaxed) {