core/rawdb, ethdb/pebble: avoid fsync db in tests

[holiman]

Proof of concept alternative to #27768 .
I don't have a mac, so can't check if it works or not.

[jsvisa]

let me have a test :)

[jsvisa]

the testing latency reduced from 47s to 20s in may mac air m1, and the flamegraph is similar to leveldb's(without the time-window of vfs)

[karalabe]

Master:

% go test -v ./core --run=TestShortRepairWithSnapshots
=== RUN   TestShortRepairWithSnapshots
--- PASS: TestShortRepairWithSnapshots (0.35s)

---

PR:

% go test -v ./core --run=TestShortRepairWithSnapshots
=== RUN   TestShortRepairWithSnapshots
--- PASS: TestShortRepairWithSnapshots (0.16s)

---

I.e. Better, but there still seems to be some perf hit somewhere

[rjl493456442]

I think we should just implement the missing feature in pebble.

[holiman]

the missing feature in pebble.

Which feature are you talking about? I think the sync/fsync method that they use is highly intentional