Add memory leak detection during server process execution

[ochaplashkin]

Memory leak detection may occur when working with the server:

g.test_foo = function()
    ...
    g.server:exec(function()
        -- there is an error from LeakSanitizer in `stderr` here
    end)
    g.server:exec(function()
        -- server is alive
   end)
   t.assert(true)
end)

g.after_all(function()
    g.server:drop() -- `test_foo` has passed
end)

We have added a check for the LeakSanitizer substring in stderr of the server process. If it's found, an error will be thrown and test will fail.

Closes #349

ℹ️ P.S. Although this PR solves the problem described in #349, there may be a more elegant solution to such detections. It is described in more detail in #358