Don't assume default constructor for thread::id is minimal value.

Fix unittest for thread id operator < to not assume thread ids are
always larger than the default in strict weak ordering. This allows for
pthread_t to be signed like it is on linux.

Change-Id: Ia4ba05e7644d3705a2f66976677e48e6939cc2bd
Merge pull request #44 from awong-dev/thread_lt_test

Don't assume default constructor for thread::id is minimal value.