Add leak detection during server termination

Memory leak detection may occur when working with the server:

    g.test_foo = function()
        ...
        g.server:exec(function()
            -- there is an error 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 been passed
    end)

We have added a check of the `stderr` from the server for the presence
of the `LeakSanitizer` substring. If it's found, an error will be caused
(test will be failed).

Closes #349

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Add logging to unified file (gh-324).
+- Add memory leak detection during server termination (gh-349).
 
 ## 1.0.1
 

luatest/server.lua

@@ -459,6 +459,17 @@ function Server:stop()
                 )
             )
         end
+        if self.process.output_beautifier.stderr:find('LeakSanitizer') then
+            error(
+                ('Memory leak during process termination (alias: %s, workdir: %s, pid: %s)\n%s')
+                :format(
+                    self.alias,
+                    fio.basename(self.workdir),
+                    self.process.pid,
+                    self.process.output_beautifier.stderr
+                )
+            )
+        end
         log.debug('Killed server process PID ' .. self.process.pid)
         self.process = nil
     end