Add the ability to set the path to the exe file

You can set an additional argument `--executable` (string) for the
test-run. This parameter sets a fixed path to the executable file.

Example usage:

$ ./test-run.py --executable /foo/bar/tarantool - a new behavior. The
path to the executable file will be "/foo/bar/tarantool".

Closes #400

lib/__init__.py

@@ -83,7 +83,7 @@ def module_init():
 
     prepend_path(os.path.join(os.environ['TEST_RUN_DIR'], 'lib/luatest/bin'))
 
-    TarantoolServer.find_exe(args.builddir)
+    TarantoolServer.find_exe(args.builddir, executable=args.executable)
     UnittestServer.find_exe(args.builddir)
     AppServer.find_exe(args.builddir)
     LuatestServer.find_exe(args.builddir)

lib/options.py

@@ -493,6 +493,18 @@ def __init__(self):
                     $ . <(./test/test-run.py --env)
                     """))
 
+        parser.add_argument(
+                '--executable',
+                dest='executable',
+                default=None,
+                help=format_help(
+                    """
+                    Set the custom path to the Tarantool executable.
+
+                    It is useful if you need to test a custom build
+                    with a different directory structure.
+                    """))
+
         # XXX: We can use parser.parse_intermixed_args() on
         # Python 3.7 to understand commands like
         # ./test-run.py foo --exclude bar baz

lib/tarantool_server.py

@@ -694,14 +694,14 @@ def __init__(self, _ini=None, test_suite=None):
             self.current_test = caller_globals['test_run_current_test']
 
     @classmethod
-    def find_exe(cls, builddir, silent=True):
+    def find_exe(cls, builddir, silent=True, executable=None):
         cls.builddir = os.path.abspath(builddir)
         builddir = os.path.join(builddir, "src")
         path = builddir + os.pathsep + os.environ["PATH"]
         color_log("Looking for server binary in ", schema='serv_text')
         color_log(path + ' ...\n', schema='path')
         for _dir in path.split(os.pathsep):
-            exe = os.path.join(_dir, cls.default_tarantool["bin"])
+            exe = executable if executable else os.path.join(_dir, cls.default_tarantool["bin"])
             ctl_dir = cls.TEST_RUN_DIR
             ctl = os.path.join(ctl_dir, cls.default_tarantool['ctl'])
             need_lua_path = False