The `test_thread` test interacts badly with Lua's GC

[kennytm]

Repro step:

1. Run cargo test --lib test_thread.
2. Repeat 10 times.
3. There will be 50% chance the test is stuck with the "median" thread never finishing.

It seems to have problem with the GC collection step. Adding collectgarbage('stop') to the Lua test code is able to workaround this.

---

Tested using lua53-ext = "0.1.1".

$ rustc -vV
rustc 1.21.0-nightly (e26688824 2017-08-27)
binary: rustc
commit-hash: e2668882406b68739c6ed33d420358d5d710e67b
commit-date: 2017-08-27
host: x86_64-apple-darwin
release: 1.21.0-nightly
LLVM version: 4.0