fix(Test): improve the params in BenchmarkDbGrowth (#1967)

It seems some parameter settings have been changed in the past. As a
result, some tests (at least one) lose the ability to test what they
want to test.
For `BenchmarkDbGrowth`, current default value of `ValueThreshold` makes
no vlog will be created and current default value of `MemTableSize`
makes no memtable will be flushed which means no compaction and thus no
vlog gc will be performed.

## Solution
Setting `ValueThreshold` be 1024 and `MemTableSize` be 1MB in this case
seems to be fine.

Signed-off-by: SpadeA-Tang <u6748471@anu.edu.au>

Author: SpadeA-Tang

db_test.go

@@ -466,6 +466,8 @@ func BenchmarkDbGrowth(b *testing.B) {
 	opts.NumVersionsToKeep = 1
 	opts.NumLevelZeroTables = 1
 	opts.NumLevelZeroTablesStall = 2
+	opts.ValueThreshold = 1024
+	opts.MemTableSize = 1 << 20
 	db, err := Open(opts)
 	require.NoError(b, err)
 	for numWrites := 0; numWrites < maxWrites; numWrites++ {