Fail testing of tarantool when it is not built

[Totktonada]

It is convenient that test-run uses a tarantool / a tarantoolctl executables found in PATH, when we want to test a module (say, vshard). However it is undesirable behaviour, when we test tarantool itself: it is not always obvious what is going on when I forgot to build tarantool prior to run tests.

We already detect tarantool source directory:

test-run/lib/__init__.py

Lines 14 to 22 in e843552

	def setenv():
	"""Find where is tarantool dir by check_file"""
	check_file = 'src/trivia/util.h'
	path = os.path.abspath('../')
	while path != '/':
	if os.path.isfile('%s/%s' % (path, check_file)):
	os.putenv('TARANTOOL_SRC_DIR', path)
	break
	path = os.path.abspath(os.path.join(path, '../'))

So it should be easy to forbid using of an external tarantool in the case.