gh-1044: Add tests for the improved change detection in box.cfg

test/app/cfgup.result

+TAP version 13
+1..3
+ok - initial config
+ok - assign the same value to immutable key (pid_file)
+ok - attempt to change immutable key (pid_file)

test/app/cfgup.test.lua

+#!/usr/bin/env tarantool
+
+-- Testing configuration updates
+
+local tap = require('tap')
+local test = tap.test('cfg')
+test:plan(3)
+
+config = {
+	pid_file = '1.pid'	
+}
+
+local status = pcall(box.cfg, config)
+test:ok(status, 'initial config')
+
+-- Assigning the same value to immutable key which is effectively a NOP,
+-- expecting success
+status = pcall(box.cfg, {pid_file = config.pid_file})
+test:ok(status, 'assign the same value to immutable key (pid_file)')
+
+-- Now change it to a different value, must fail
+local result
+status, result = pcall(box.cfg, {pid_file = 'Z'..config.pid_file})
+test:ok(not status and
+        result:match("Can't set option 'pid_file' dynamically"),
+        'attempt to change immutable key (pid_file)')
+
+test:check()
+os.exit(0)
+