Add unit tests for fraction search

[cheb0]

Currently, most fraction search tests are integration tests which reside in tests/integration_tests/single_test.go. We need to add unit tests for fraction (active, sealed) which do not start dedicated proxy and store instances.

Scenario for tests:

• OR / AND / NOT queries
• wildcards
• limit / offset / (with and without total)
• asc/desc sort
• hists
• aggregations

We need to run tests against different suites:

• active fraction
• freshly sealed fraction
• sealed and read from disk fraction