Fix flaky `downloads_counter::tests::test_increment_missing_version` test

[Turbo87]

The new downloads_counter::tests::test_increment_missing_version test, that was introduced by #3423, appears to be flaky, and sometimes causes our CI to fail:

---- downloads_counter::tests::test_increment_missing_version stdout ----
thread 'downloads_counter::tests::test_increment_missing_version' panicked at 'assertion failed: `(left == right)`
  left: `PersistStats { shard: None, counted_downloads: 1, counted_versions: 1, pending_downloads: 1 }`,
 right: `PersistStats { shard: None, counted_downloads: 1, counted_versions: 1, pending_downloads: 0 }`', src/downloads_counter.rs:379:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/panicking.rs:92:14
   2: cargo_registry::downloads_counter::tests::test_increment_missing_version
             at ./src/downloads_counter.rs:379:9
   3: cargo_registry::downloads_counter::tests::test_increment_missing_version::{{closure}}
             at ./src/downloads_counter.rs:361:5
   4: core::ops::function::FnOnce::call_once
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/ops/function.rs:227:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/ops/function.rs:227:5

/cc @pietroalbini