python3: --builddir regression when a relative path is passed

[Totktonada]

$ cd path/to/tarantool/repo
$ mkdir b && cd b
$ cmake .. -DCMAKE_BUILD_TYPE=Debug -DENABLE_BACKTRACE=ON -DENABLE_DIST=ON -DENABLE_BUNDLED_LIBCURL=OFF && make -j
$ python2 ../test/test-run.py --builddir=../b unit/guard
<...>
[001] unit/guard.test                                                 [ pass ]
$ python3 ../test/test-run.py --builddir=../b unit/guard
<...>
[001] unit/guard.test                                                 
[001] Test.run() received the following error:
[001] Traceback (most recent call last):
[001]   File "/home/alex/projects/tarantool-meta/r/t-3/test-run/lib/test.py", line 190, in run
[001]     self.execute(server)
[001]   File "/home/alex/projects/tarantool-meta/r/t-3/test-run/lib/unittest_server.py", line 18, in execute
[001]     proc = Popen(execs, cwd=server.vardir, stdout=PIPE, stderr=STDOUT)
[001]   File "/usr/lib/python3.9/subprocess.py", line 947, in __init__
[001]     self._execute_child(args, executable, preexec_fn, close_fds,
[001]   File "/usr/lib/python3.9/subprocess.py", line 1819, in _execute_child
[001]     raise child_exception_type(errno_num, err_msg, err_filename)
[001] FileNotFoundError: [Errno 2] No such file or directory: '/home/alex/projects/tarantool-meta/r/t-3/b/b/test/unit/guard.test'
[001] 
[001] [ fail ]
<...>
$ python3 ../test/test-run.py --builddir=$(realpath ../b) unit/guard
[001] unit/guard.test                                                 [ pass ]