Add coverage stage in CI for MRs

We need to take advantage of coverage gathering for tarantool and picodata. The issue was moved from tarantool repo.

Previous content:

At the moment we don't run full tests for each MR - only luacheck, test-ubuntu, test_mac-m1 and checkpatch steps are executed. But there is also the coverage step with lot's of additional test that is executed only on release step. It is incorrect because we can miss some errors (as happened after picodata/tarantool@86b5ef4c) and I don't like it: we can merge only those MRs that are absolutely correct.