Save server artifacts once regardless of calls

Due to the specifics of working with the test and the added connection
between `test <-> server` as well as because artifacts will be saved
with a connection problem [1], the `Server:save_artifacts()` method were
called more than once. In case of any problems we could overwrite already
saved artifacts. Added a flag that will ensure that the saving will be
executed only once.

There was also a problem when copying artifacts was not performed but the
path to the artifacts was formed as a directory with artifacts (as if it
exists):

    artifacts:
        server -> /tmp/t/artifacts/server-XXX

And if we try to look at these artifacts, nothing will work:

    $ ls -la /tmp/t/artifacts/server-XXX
    ls: cannot access '/tmp/t/artifacts/server-XXX': No such file or
    directory

To show explicitly that the saving failed with an error, the string will
now be written to the artifacts:

    artifacts:
        server -> Failed to copy artifacts for server (workdir:
        /tmp/t/artifacts/server-XXX)

[1] tarantool/luatest@251b35f

Close tarantool#304

luatest/runner.lua

@@ -338,6 +338,11 @@ function Runner.mt:update_status(node, err)
     elseif err.status == 'fail' or err.status == 'error' or err.status == 'skip'
         or err.status == 'xfail' or err.status == 'xsuccess' then
         node:update_status(err.status, err.message, err.trace)
+        if utils.table_len(node.servers) > 0 then
+            for _, server in pairs(node.servers) do
+                server:save_artifacts()
+            end
+        end
     else
         error('No such status: ' .. pp.tostring(err.status))
     end
@@ -458,13 +463,6 @@ end
 function Runner.mt:invoke_test_function(test)
     local err = self:protected_call(test.group, test.method, test.name)
     self:update_status(test, err)
-    if not test:is('success') then
-        if utils.table_len(test.servers) > 0 then
-            for _, server in pairs(test.servers) do
-                server:save_artifacts()
-            end
-        end
-    end
 end
 
 function Runner.mt:find_test(groups, name)

luatest/server.lua

@@ -371,14 +371,21 @@ function Server:restart(params, opts)
     log.debug('Restarted server PID: ' .. self.process.pid)
 end
 
--- Save server artifacts by copying the working directory.
--- Throws an error when the copying is not successful.
+--- Save server artifacts by copying the working directory.
+-- The save logic will only work once to avoid overwriting the artifacts directory.
+-- If an error occurred, then server artifacts path will be replaced follow string:
+-- `Failed to copy artifacts for server (alias: <alias>, workdir: <workdir>)`.
 function Server:save_artifacts()
+    if self.is_artifacts_handled then
+        return
+    end
     local ok, err = fio.copytree(self.workdir, self.artifacts)
     if not ok then
-        log.error(('Failed to copy artifacts for server (alias: %s, workdir: %s): %s')
-            :format(self.alias, fio.basename(self.workdir), err))
+        self.artifacts = ('Failed to copy artifacts for server (alias: %s, workdir: %s)')
+                        :format(self.alias, fio.basename(self.workdir))
+        log.error(('%s: %s'):format(self.artifacts, err))
     end
+    self.is_artifacts_handled = true
 end
 
 -- Wait until the given condition is `true` (anything except `false` and `nil`).