Fix decoding error when reading server's log file

When a diff test starts a Tarantool server, test-run reads the server's
log file and tries to find the message indicating the server is ready.
Sometimes the server's log file may contain bytes that cannot be decoded
by `utf-8` codec and test-run fails with an error like this:

    [044] TarantoolInpector.handle() received the following error:
    [044] Traceback (most recent call last):
    [044]   File "/tarantool/test-run/lib/inspector.py", line 98, in handle
    [044]     result = self.parser.parse_preprocessor(line)
    [044]   File "/tarantool/test-run/lib/preprocessor.py", line 123, in parse_preprocessor
    [044]     return self.server(stype, sname, options)
    [044]   File "/tarantool/test-run/lib/preprocessor.py", line 351, in server
    [044]     return getattr(self, attr)(ctype, sname, opts)
    [044]   File "/tarantool/test-run/lib/preprocessor.py", line 209, in server_start
    [044]     self.servers[sname].start(silent=True, rais=True, wait=wait,
    [044]   File "/tarantool/test-run/lib/tarantool_server.py", line 910, in start
    [044]     self.wait_until_started(wait_load, deadline)
    [044]   File "/tarantool/test-run/lib/tarantool_server.py", line 1147, in wait_until_started
    [044]     self.wait_load(deadline)
    [044]   File "/tarantool/test-run/lib/tarantool_server.py", line 1131, in wait_load
    [044]     if not self.logfile_pos.seek_wait(msg, p, self.name, deadline):
    [044]   File "/tarantool/test-run/lib/tarantool_server.py", line 485, in seek_wait
    [044]     log_str = f.readline()
    [044]   File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/codecs.py", line 322, in decode
    [044]     (result, consumed) = self._buffer_decode(data, self.errors, final)
    [044] UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf0 in position 660: invalid continuation byte

At least, I have seen such errors when Tarantool with JIT enabled was
tested on macOS ARM64 machines. So this patch fixes the issue.

lib/tarantool_server.py

@@ -45,6 +45,7 @@
 from lib.utils import warn_unix_socket
 from lib.utils import prefix_each_line
 from lib.utils import prepend_path
+from lib.utils import PY3
 from lib.test import TestRunGreenlet, TestExecutionError
 
 
@@ -440,17 +441,27 @@ def __init__(self, path):
         self.path = path
         self.log_begin = 0
 
+    def open(self, mode, **kwargs):
+        if PY3:
+            # Sometimes the server's log file may contain bytes that cannot be
+            # decoded by utf-8 codec and test-run fails with an error like this:
+            #     UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf0 in
+            #         position 660: invalid continuation byte
+            # The option below fixes it. Note, Python2 doesn't know the option.
+            kwargs['errors'] = 'replace'
+        return open(self.path, mode, **kwargs)
+
     def positioning(self):
         if os.path.exists(self.path):
-            with open(self.path, 'r') as f:
+            with self.open('r') as f:
                 f.seek(0, os.SEEK_END)
                 self.log_begin = f.tell()
         return self
 
     def seek_once(self, msg):
         if not os.path.exists(self.path):
             return -1
-        with open(self.path, 'r') as f:
+        with self.open('r') as f:
             f.seek(self.log_begin, os.SEEK_SET)
             while True:
                 log_str = f.readline()
@@ -472,7 +483,7 @@ def seek_wait(self, msg, proc=None, name=None, deadline=None):
                          "seconds\n".format(self.path, timeout), schema='error')
             return False
 
-        with open(self.path, 'r') as f:
+        with self.open('r') as f:
             f.seek(self.log_begin, os.SEEK_SET)
             cur_pos = self.log_begin
             while not deadline or time.time() < deadline:
@@ -979,7 +990,7 @@ def crash_grep(self):
         # find and save backtrace or assertion fail
         assert_lines = list()
         bt = list()
-        with open(self.logfile, 'r') as log:
+        with self.logfile_pos.open('r') as log:
             lines = log.readlines()
             for rpos, line in enumerate(reversed(lines)):
                 if line.startswith('Segmentation fault'):