Enable logging to unified file

If the test fails and there is a unified log file (`run.log` by default)
you'll see the following output:

 - [box-luatest/varbinary_test.lua, null]
  # logfile:         /tmp/t/log/031_box-luatest.log
  # reproduce file:  /tmp/t/reproduce/031_box-luatest.list.yaml
  # unified logfile: /tmp/t/031_box-luatest/run.log

Closes #427

lib/luatest_server.py

@@ -34,10 +34,10 @@ def __init__(self, *args, **kwargs):
     def execute(self, server):
         """Execute test by luatest command
 
-        Execute `luatest -c --no-clean --verbose <name>_test.lua --output tap`
-        command. Disable capture mode and deletion of the var directory.
-        Provide a verbose output in the tap format. Extend the command by
-        `--pattern <pattern>` if the corresponding option is provided.
+        Execute `luatest -c --no-clean -l <path> --verbose <name>_test.lua --output tap`
+        command. Disable capture mode and deletion of the var directory. Provide a verbose
+        output in the tap format. Extend the command by `--pattern <pattern>` if the
+        corresponding option is provided.
         """
         server.current_test = self
         script = os.path.join(os.path.basename(server.testdir), self.name)
@@ -51,6 +51,8 @@ def execute(self, server):
         if Options().args.pattern:
             for p in Options().args.pattern:
                 command.extend(['--pattern', p])
+        unified_logfile_path = os.path.join(server.vardir, 'run.log')
+        command.extend(['-l', unified_logfile_path])
 
         # We start luatest from the project source directory, it
         # is the usual way to use luatest.

lib/worker.py

@@ -69,6 +69,16 @@ def get_reproduce_file(worker_name):
     return os.path.join(reproduce_dir, '%s.list.yaml' % worker_name)
 
 
+def get_unified_logfile(worker_name):
+    # Unified logging is only for LuatestServer workers.
+    # It doesn't always guarantee a log file.
+    # If it doesn't exist, it will return None.
+    if 'luatest' in worker_name:
+        path = os.path.join(main_vardir(), worker_name, Options().args.log)
+        if os.path.isfile(path):
+            return path
+
+
 def print_greetings():
     # print information about tarantool
     color_stdout('\n')

listeners.py

@@ -14,6 +14,7 @@
 from lib.worker import WorkerOutput
 from lib.worker import WorkerTaskResult
 from lib.worker import get_reproduce_file
+from lib.worker import get_unified_logfile
 from lib.utils import prefix_each_line
 from lib.utils import safe_makedirs
 from lib.utils import print_tail_n
@@ -167,6 +168,9 @@ def print_tasks_info(self, tasks):
                 color_stdout("---\n", schema='separator')
                 print_tail_n(reproduce_file_path)
                 color_stdout("...\n", schema='separator')
+            unified_logfile_path = get_unified_logfile(worker_name)
+            if unified_logfile_path:
+                color_stdout('# unified logfile: %s\n' % unified_logfile_path)
 
     def print_statistics(self):
         """Print statistics and results of testing."""