python3: make path to unit test absolute

When Tarantool unit tests runs using test-run error with output like below
may happen:

[027] small/small_class.test
[027] Test.run() received the following error:
[027] Traceback (most recent call last):
[027]   File "/__w/tarantool/tarantool/test-run/lib/test.py", line 192, in run
[027]     self.execute(server)
[027]   File "/__w/tarantool/tarantool/test-run/lib/unittest_server.py", line 20, in execute
[027]     proc = Popen(execs, cwd=server.vardir, stdout=PIPE, stderr=STDOUT)
[027]   File "/usr/lib/python3.5/subprocess.py", line 676, in __init__
[027]     restore_signals, start_new_session)
[027]   File "/usr/lib/python3.5/subprocess.py", line 1282, in _execute_child
[027]     raise child_exception_type(errno_num, err_msg)
[027] FileNotFoundError: [Errno 2] No such file or directory: '../test/small/small_class.test'
[027] [ fail ]

The root cause of error is changed behaviour of Popen in Python 3 in comparison
to Python 2. One should explicitly set path to a current work dir:

Python 2 [1]: "If cwd is not None, the child’s current directory will be
changed to cwd before it is executed. Note that this directory is not
considered when searching the executable, so you can’t specify the
program’s path relative to cwd."

Python 3 [2]: "If cwd is not None, the function changes the working
directory to cwd before executing the child. <...> In particular, the
function looks for executable (or for the first item in args) relative
to cwd if the executable path is a relative path."

1. https://docs.python.org/2/library/subprocess.html#subprocess.Popen
2. https://docs.python.org/3/library/subprocess.html#subprocess.Popen

Part of #20

lib/unittest_server.py

@@ -48,7 +48,7 @@ def binary(self):
     def prepare_args(self, args=[]):
         executable_path = os.path.join(self.builddir, "test",
                                        self.current_test.name)
-        return [executable_path] + args
+        return [os.path.abspath(executable_path)] + args
 
     def deploy(self, vardir=None, silent=True, wait=True):
         self.vardir = vardir