test: disable app/fiber.test.lua on macOS/M1

Regular testing routine on macOS/M1 platforms will be enabled back in
the last patch of the series, but there are some failures unrelated to
the changes made within the previous commits. Hence it was decided to
add the skipcond unit to app/fiber.test.lua for now.

Relates to tarantool/tarantool-qa#308

NO_DOC=tests
NO_CHANGELOG=tests

test/app/fiber.skipcond

@@ -4,4 +4,8 @@ import platform
 if platform.system() == 'OpenBSD':
     self.skip = 1
 
+# Disabled on macOS/M1 due to fail tarantool/tarantool-qa#308.
+if platform.machine() == 'arm64' and platform.system() == 'Darwin':
+    self.skip = 1
+
 # vim: set ft=python: