tests: add ability to run box/engine tests with different allocators

The ability to select an allocator for memtx has been added to
tarantool. To test a new type of allocator, all tests must also
be run with it. Implemented new option, which allows to set allocator
for memtx. If you wan't to choose allocator type for tests, run test-run.py
with --memtx-allocator="small" or --memtx-allocator="system". Allocator type
is passed via MEMTX_ALLOCATOR environment variable to the test.