Improve tests and benchmarks for insert

[b1ron]

We should update/add benchmarks to test:

• concurrent inserts into the same collection;
• concurrent inserts into different collections.

The best way to do that is probably to make two copies of that benchmark and change them:

FerretDB/integration/benchmarks_test.go

Line 133 in 2664b9a

func BenchmarkInsertMany(b *testing.B) {

[wazir-ahmed]

@AlekSi I'm interested in picking up this task next.