Fix detect tarantool crash at exit

> What's the problem?

During the shutdown of the server, the server process is polled. If the
call `ffi.C.kill(<pid>, 0)` returns the value 0, then the process will
be considered alive, otherwise it will be dead.

Luatest cannot distinguish the states when the process is completed
correctly and incorrectly. This means that if a critical error appears
at the end of the process (e.g., segmentation fault), then stopping the
server will be considered successful (and the test-case too).

> What's the solution?

Each process contains its own instance of output_beautifier (which reads
stdout and stderr). Now this instance will contain the stderr field (str)
where the entire stderr from the process will be saved.

During the server shutdown, it looks for the substring 'segmentation fault'
and return an error if the substring is found:

	Segmentation fault during process termination (alias: master,
		workdir: master-SrQjEIiLyRHS, pid: 152323)
	Segmentation fault
  		code: SEGV_MAPERR
  		addr: (nil)
		...

Closes #252

Author: Oleg Chaplashkin

luatest/output_beautifier.lua

@@ -70,6 +70,7 @@ function OutputBeautifier.mt:initialize()
         self.class.COLOR_BY_NAME[self.color] or
         OutputBeautifier:next_color_code()
     self.pipes = {stdout = ffi_io.create_pipe(), stderr = ffi_io.create_pipe()}
+    self.stderr = ''
 end
 
 -- Replace standard output descriptors with pipes.
@@ -86,7 +87,7 @@ function OutputBeautifier.mt:enable(options)
     end
     self.fibers = {}
     for i, pipe in pairs(self.pipes) do
-        self.fibers[i] = fiber.new(self.run, self, pipe[0])
+        self.fibers[i] = fiber.new(self.run, self, pipe[0], i)
     end
     self.fibers.pid_tracker = options and options.track_pid and fiber.new(function()
         Process = Process or require('luatest.process')
@@ -138,12 +139,16 @@ end
 --
 -- Every line with log level mark (` X> `) changes the color for all the following
 -- lines until the next one with the mark.
-function OutputBeautifier.mt:run(fd)
+function OutputBeautifier.mt:run(fd, pipe)
     local prefix = self.color_code .. self.prefix .. ' | '
     local line_color_code = self.class.RESET_TERM
     while fiber.testcancel() or true do
         self:process_fd_output(fd, function(chunks)
-            local lines = table.concat(chunks):split('\n')
+            local raw_lines = table.concat(chunks)
+            if pipe == 'stderr' then
+                self.stderr = self.stderr .. raw_lines
+            end
+            local lines = raw_lines:split('\n')
             if lines[#lines] == '' then
                 table.remove(lines)
             end

luatest/server.lua

@@ -367,6 +367,17 @@ function Server:stop()
         if not ok and not err:find('Process is terminated when waiting for') then
             error(err)
         end
+        if self.process.output_beautifier.stderr:find('Segmentation fault') then
+            error(
+                ('Segmentation fault during process termination (alias: %s, workdir: %s, pid: %d)\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