Add additional tests for GetKeysByPrefix API

One sstable per level

We need to test the API works by combining the results from multiple levels of the LSMTree. For this we should have a test that has a sstable per level.

We will need a compactlevel API that does a force compaction from a level to the next level, irrespective of all the triggers. We need to have this on a Magma level exposed via MagmaAPIUtil, for testing purposes.

prefix bloom filter

In prefix mode, the bloom filters for every sstable only store the prefix of the keys. We should test that. We can mimic the test done in kvstore_test.cc

TEST_F(MagmaKVStoreTest, key_may_exist) {